Transcript for:
Basic Chemistry for Life

hey everyone Dr D here and in this video we are going to be covering chapter two from our Campbell Biology 12th edition so let's go ahead and get started [Music] explain stuff now you may be wondering why are we learning chemistry which is the topic of Chapter two The Chemical Context of Life why are we learning chemistry in a biology class and that's a good question the answer is really in that biology for science Majors one uh biology 1406 for my students is a essentially a Cell Biology class this means that we are going to be studying things that are no bigger than a cell we are we are really interested in how the functional unit of a cell works right so remember that the cell is the basic unit of life and all living things are made up of at least one cell so if you understand how a cell works then you'll understand how pretty much all life works because life is based on the basic unit of life which is a cell okay so this this is why just as a Prelude this is why we're learning about chemistry because think about what's happening inside of a cell you've got these organelles like the mitochondria the chloroplasts the nucleus DNA and all of these are based on molecular interactions that make them work so if you want to understand how these organelles work and how the molecules work inside and how the proteins interact with one another and how DNA works and how RNA works well then you really need to know your Chemistry you need to understand molecular bonds you need to understand molecular interactions and this is why we're studying chemistry in chapter two so believe me it is important that you understand these Concepts from chapter two it will make the rest of the semester that much easier so let's Dive Right into Chapter two The Chemical Context of life and the basics of chemistry are that everything is made up of matter organisms are also made up of matter matter is anything that takes up space and has mass and if you want to know what makes up matter just think of the periodic table of elements the periodic table of elements this is what makes up matter matter is made up of the elements if you have an absence of matter by the way that's a vacuum so think of the vacuum of space outer space where there is no air because even even air even air is matter right so when you're when you're flailing your arms around you're you're you're going through matter you're going through oxygen you're going through nitrogen you're going through carbon dioxide and other gases right so so this is this is matter uh the the opposite of matter would be a vacuum the absence of any substance so what is an element remember the periodic table of elements you got hydrogen helium helium lithium Boron beryllium Boron Etc an element is a substance that cannot be broken down to other substances by chemical reactions okay so those are the elements that as they appear on the periodic table of elements what is a compound then a compound is a substance consisting of two or more elements but they have to be at a fixed ratio think of think of sodium chloride right NaCl as a great example of a compound and when compounds form what you should know is that when a compound form it has characteristics that are different from it's it it's individual elements that comprise the compound this is a type of emergent property remember we learned about emergent properties in chapter one that these traits appear that are greater than the sum of the parts so let me show you what I'm talking about here's the compound I'm talking about sodium chloride table salt this is the yummy substance that you sprinkle on your French fries to make them tasty okay but what is this yummy substance uh comprised of it's comprised of sodium and chlorine so let's take a look at sodium pure sodium and chlorine pure chlorine sodium did you know that sodium is a metal it is a it is a metal that is quite reactive did you know if you throw sodium metal into water it starts it starts popping it starts kind of kind of exploding okay so it's it it violently reacts with water so it's a it's it's a it's kind of a very highly reactive exploding metal when added to water okay so this is sodium chlorine exists as a gas okay chlorine is a gas and not only is it a gas but it is a it is a toxic hazardous gas you do not want to breathe chlorine gas okay it is quite poisonous but look at this when sodium reacts with chlorine to form the compound sodium chloride it becomes a yummy table salt so um the the yummy table salt is an emergent property is it not it is different than the sum of its parts an explosive metal and a toxic gas come together to form a yummy seasoning you know so this is an emergent property of this compound and compounds have these emergent properties that are different than the sum of the individual elements that make them up now on the periodic table of elements 92 are natural elements and of those 92 elements 20 to 25 percent are required for life these are known as essential elements for life and of those elements only four wicket there's Wicket in the background only four make up 96 of living matter carbon hydrogen oxygen and nitrogen these four elements alone make up 96 of your body they make up 96 percent of the living matter on the Earth most of the remaining four percent consist of calcium phosphorus potassium and sulfur there are some elements that are required in minute quantities tiny quantities these are known as the trace elements for example do you guys know that you need iron in your blood right everyone knows that you have iron in your blood well iron is a trace element it is required in relatively minut quantities here we can see the elements in the human body and do you remember just these four elements oxygen carbon hydrogen nitrogen those four elements alone make up 96 of the human body and why is that well we're going to learn this in the next chapter but these are the elements that are found in your biomolecules your proteins your carbohydrates your lipids and your DNA and your RNA the nucleic acids next we have calcium phosphorus potassium sulfur sodium chlorine and magnesium this is what rounds out the rest of the approximately four percent of your human body but do you remember there's also a host of those Trace elements that you only need in minute quantities these include Boron chromium copper iodine iron I'm not going to list them all but these are these are elements that your body requires in tiny amounts this is why they're called Trace elements now each element consists of unique atoms an atom is the smallest unit of matter that still retains the properties of an element think of an atom as an individual piece of an element and a molecule then is a group of atoms bonded together and what is an atom comprised of atoms are actually made up of smaller parts called subatomic particles and there are three main subatomic particles there are neutrons which have no electrical charge there are protons which have a positive charge and the tiny electrons which have a negative charge these are the three subatomic particles that make up atoms now near the center of each atom there is what's known as the atomic nucleus this is where the neutrons and the protons lie the neutrons and the protons have a mass of what's called a Dalton both neutrons and protons weigh about a Dalton the Dalton is the mass of a individual Proton or Neutron now electrons they don't exist in the atomic nucleus instead electrons form a cloud of negative charge around the nucleus and those electrons spend time in these clouds most of their time in these clouds we'll talk about this in a minute and electrons they don't have a mass of a Dalton they are so small compared to a neutron or a proton that they are ignored their masses largely ignored when calculating the total mass of an atom in fact an electron is about 2 000 times smaller than either a neutron or a proton electrons are tiny here is a simplified model of what helium the element helium looks like here's an atom of helium depicted two different ways on the left you see that helium in its Atomic nucleus here in the center has two positively charged protons and two neutral in charge neutrons notice how they drew the neutrons and the protons roughly the same size that makes sense remember because the protons and the neutrons have the same mass so they are the same size and so this would be a four Dalton Atomic nucleus and then the electrons remember the electrons are so small that you wouldn't be able to see them because there would be 2 000 times smaller than these protons or neutrons and those electrons exist anywhere in this cloud and helium has two electrons in the model on the right you can see how many electrons helium has helium has two electrons so here you can see that there's two electrons that live in this cloud now one more very important thing about atoms I need to share with you an atom as it appears on the periodic table of elements by definition an atom has the same number of protons as it does electrons this is because an atom doesn't have a overall net charge for an element to not have an overall net charge each positive charge of a proton needs to be canceled out by a negative charge of an electron does that make sense so atoms on the periodic table and atoms in general have no charge why would they have no charge well that means no net charge for every positive proton an atom has a negatively charged electron so if helium atom has two protons helium atom must also have two electrons so just remember that is very important that you remember that atoms are neutral in charge that means they have the same number of protons as they do electrons now atoms of the various elements differ in the number of their subatomic particles so have you ever seen the periodic table of elements have you ever looked at an individual element let's take carbon for instance the smaller number is known as the atomic number this is the number of protons in the atomic nucleus of that atom okay the smaller number is the atomic number it is the number of protons and by the way the periodic table of elements is arranged by increasing numbers of uh atomic number of protons with hydrogen at the top left having one proton and helium at the top right having two protons and then if we go to the next row kind of like you're reading a book if we go to the next row you have lithium with three protons its atomic number is three you have right next to it beryllium with four protons its atomic number is four and then you have Boron with five and then carbon with six nitrogen with seven oxygen with eight fluorine with nine and finally neon with ten you see how it works the periodic table of elements is arranged with elements in increasing atomic number from left to right from top to bottom and that's what identifies an element I want to share with you that carbon has six protons no other element has six protons so by definition you can identify an atom based on the number of protons it has there if if the element has six protons it is carbon period the end okay there's no other element that would have any business having six protons carbon can have strange numbers of neutrons and still be carbon carbon can have strange numbers of electrons and still be carbon but but carbon cannot have anything other than six protons and B carbon this is why we know that the atomic number is the number that's used to identify an element I hope that makes sense let's carry on so what is the larger number on the uh on the on the periodic table when you look at that chemical symbol the larger number is known as the mass number and it's a little tricky because it's the sum of the protons plus the neutrons in the nucleus so again if we look at Carbon remember the atomic number is six which means that's the number of protons it has but the larger mass number is 12 in regular carbon 12. the the larger number is 12 but remember that doesn't mean carbon has 12 neutrons because 12 is the sum of the protons plus the neutrons therefore you have to subtract the number of protons the atomic number from the mass number the the number of protons plus the number of neutrons so how many neutrons does carbon possess the answer is 12 minus 6 or 6. okay I hope that makes sense again the atomic number is the number of protons and the mass number is the number of protons plus the number of neutrons in that nucleus now the atomic mass is the atoms total mass which is not just the number of protons and the number of neutrons which is the mass number but that also includes the mass of the electrons but remember I told you that the electrons are so teeny tiny they are 2 000 times smaller than an individual proton or a neutron so that it's negligible it doesn't even matter it's it's it can be approximate the atomic mass can be approximated by just looking at the mass number the the num the mass of the electrons really doesn't add that much now one more thing I want to share with you that is really fascinating is that the atomic nucleus makes up most of the mass of the atom however the electron cloud you know where the electrons Live In This Cloud that takes up most of the space of an atom do you see what I'm talking about the protons and the neutrons are massive they're in the center of the atom however the electrons are tiny but they take up so much space so let me explain how this works imagine if the protons and the neutrons the atomic nucleus was the size of this pencil eraser right here you see right in my hand I have this pencil eraser and this is the mass of the protons and the neutrons so you're looking at the protons and the neutrons of a typical element then imagine if I took this pencil eraser and I placed it in the center of Cowboys Stadium okay now bear with me here now I place the Eraser in the 50-yard line of Cowboy stadium guess how much space the electron cloud takes up with those tiny electrons and by the way you wouldn't even be able to see those tiny electrons because it would be 2 000 times smaller than my little pencil eraser here well guess what those electrons would be buzzing around in this cloud and the cloud would be would take up the rest of that Cowboy stadium isn't that interesting so think about that if you saw an atom the size of Cowboy stadium coming at you it's like it's like a giant atom the size of Cowboy stadium and it's coming right at you do you know what you would see you wouldn't see hardly anything actually you would just see this little pencil eraser 50 yards away that that that is an atom so think about this for a second an atom is almost entirely empty space isn't it because think about it if an atom and the size of Cowboy stadium all you can see if you squint really hard is a little pencil eraser which is the mass of the atomic nucleus that means most of the atom is just empty space isn't that fascinating most of matter is empty space most of an atom is empty space this is why when you go and you need an X-ray done they shoot x-rays through your body those x-rays go through your body never having hit anything and they come out the other side and get developed on that x-ray film is that not fascinating those x-rays travel right through your body and don't touch a thing and your bones appear brighter and this is because your bones are more dense and your bones absorb more of the x-rays isn't that interesting so the bones absorb the x-rays because they are more dense but the x-rays pass right through your soft tissues and your organs is that not fascinating and that's because you are mostly empty space so everything is made up of mostly empty space now all atoms of a particular element have the same number of protons for instance carbon always has six protons however the number of neutrons can actually vary so for example there is carbon 12 which has six neutrons carbon 13 which has seven neutrons and carbon 14 has eight neutrons and these elements that have different numbers of neutrons are called Isotopes so carbon 12 13 and 14 are isotopes they are atoms that have different numbers of neutrons and sometimes this causes problems this can lead to radioactive isotopes some of these Isotopes are radioactive they Decay spontaneously and as they Decay they give off particles and energy and eventually radioactive isotopes break apart they kind they Decay a parent isotope decays into its daughter isotope at a fixed rate this rate is known as the half-life of the isotope so different radioactive elements have different half-lives some range in the number of days or weeks and some can range into thousands of years depending on the element that's radioactive and the reason why elements become radioactive is because the number of neutrons and protons in the atomic nucleus if they're not in the correct ratio and those protons are exposed to one another or get too close to one another this can cause the atomic nucleus to flux and this can cause the the the protons to want to get away from one another and that causes that instability that causes the gamma radiation the beta radiation all the radioactive emissions to occur isn't that neat that's the premise behind radioactivity and scientists use this uh this a half-life this knowledge of half-lives to do radiometric dating where scientists measure the ratio of different isotopes and calculate how many half-lives have passed since the particular fossil or Rock was formed this can allow you to date fossils and say well this fossil it's about you know half a million years old whereas this fossils 12 million years old etc etc so now I'm going to share with you some facts about the electrons of an atom which won't be very clear and won't be very understandable until we turn our attention to photosynthesis in a subsequent chapter okay so just keep in mind when we talk about photosynthesis these concepts of how electrons store energy and the energy level of electrons is gonna reappear so for now it might be a little confusing but bear with me in this explanation of how energies energy is associated with the electrons of an atom so first of all I just want to Define energy energy is the capacity to cause change and there's a form of energy known as potential energy which is the energy that matter possesses because of its location or structure and matter has a natural tendency to move towards the lowest possible state of potential energy now this potential energy is applicable to the electrons of an atom because those electrons of an atom differ in their amounts of potential energy based on what based on their distance from the nucleus and these changes in potential energy of electrons can occur only in steps of fixed amounts and these fixed amounts are dictated by what are known as electron shells electrons found in different electron shells each with a characteristic average distance and energy level so let me show you what I'm talking about there's there's here you can see this is the atomic nucleus of an atom uh electrons can exist in what is known as the first shell this shell remember that cloud of electrons there'll be electrons in this shell and those are known as the lowest energy electrons then we go to the next Shell you can think of a shell as a layer like a layer of electrons and you see the next Shell the next layer of electrons the next bigger Cloud you can think of it that way these are higher energy electrons and then the third shell is even further away from the atomic nucleus these have yet more energy in these electrons and if an electron is pushed from a more inner shell to an outer shell that means that energy has been absorbed and if those electrons drop from an outer shell to an inner shell energy is released okay so again I know this is a little tricky and not very intuitive but keep this concept in mind for photosynthesis where this will become perfectly uh you know a perfectly good explanation of how chlorophyll captures sunlight now let me explain something very important about an atom and this is with regard to the electrons do you guys remember how I mentioned that the electrons live in a cloud uh quite far from the atomic nucleus so if this is the atomic nucleus right here these are the protons and the neutrons of the cell I said that the electrons exist In This Cloud that takes up a lot of space remember the rest of Cowboy stadium outside of that but that's only half the story what you need to know is that only two electrons can exist in any one three-dimensional space and these three dimensional spaces are called orbitals so again two electrons for every orbital an orbital is a three-dimensional space but do you remember the other term we learned about electrons and where they live they were called shells do you remember that term a shell what I need you to know is that a shell and an orbital are not the same thing a shell is a layer remember like a layer of cake you had the first shell then you have a second shell and you have a third shell and remember the shells are like layers of cake they're going further and further away from the atomic nucleus so you would have the first shell the second shell the third shell and it goes on and on there's the fourth shell there's the fifth shell there's a sixth and seventh shell okay so these are called shells these are layers in which electrons can exist all right but remember orbital and shell are two different things the first shell the first shell of an atom has one orbital has one orbital so I'm going to ask you this how many electrons do you think can live in that first shell if the first shell only has one orbital and remember only two electrons can exist in any one orbital that's right the answer is two and I'm going to draw the electrons in Red so we can keep track so the first shell the first layer contains two electrons I'm just going to draw them like this two electrons exist in the first layer of cake all right in the first shell because the first shell is only one three-dimensional space and that three-dimensional space looks like a ball all right looks like a sphere and it's called the 1s orbital in case you're wondering now because we filled that layer now we've filled that layer with electrons we have to kick it out to the next layer right we have to go from Shell one we gotta kick it out to Shell two but here's the trick to Shell two shell two has actually not one orbital it has four orbitals it has one orbital that looks like a sphere it's called the 2s orbital but it also has an additional three orbitals that look kind of like the infinity sign they're called dumbbell shaped orbitals or the P orbitals so if that's true let me ask you this if that's true the second shell if the second shell possesses four orbitals four three-dimensional shapes in which electrons can live how many electrons can exist in the second shell that's right because there's four orbital and each orbital can hold two electrons in that three-dimensional space it's eight so we're going to draw eight little dots for our eight electrons in the second shell so the second shell can house eight total electrons before becoming totally full all right great now if we have eight electrons in the second shell and two electrons in the first shell now if an atom has even more electrons guess what we have to do with those extra electrons we have to kick it out to the third shell now we've got to kick it out again to the third shell and and guess what the third shell also has four orbitals those four orbitals look like a inflated version of the second shell remember the second shell I said it had four orbitals a spherical orbital and three dumbbell shaped orbitals well the third or the third shell also has a big ball a big sphere called the 3s orbital and it has three big dumbbells right so if that's true again if that's true and the third layer of cake the third shell also has four orbitals four three-dimensional spaces how many electrons can exist in the third shell that's right it's another eight so another eight electrons can exist in the third shell all right check that out and now you have filled the third shell so if an if an atom had even more electrons guess what we'd have to do that's right we'd have to kick it out to the next Shell the fourth shell and that's how electrons work electrons exist in these orbitals and these shells and those orbitals and shells they fill from the inside out and the more electrons an atom possesses the more shells it's going to have right so so the bigger the element is the the heavier the element is the more Pro the more protons an element has that means it has more electrons so the bigger the element with the more protons you're going to have more electrons and you're going to have more shells right so this is how it works and by the way I need to define a term for you real quick whenever we're addressing the outer shell whenever we're talking about the outermost shell that is called the valence shell so if there was one electron look at this if there was one electron in the fourth shell and I asked you how many valence electrons does this atom possess you would say one right because it's outermost shell it's Fourth shell contains one lonely electron so this this atom has one valence electron so just keep in mind valence electrons are the outermost shell electrons now coming back to our slides check out this uh part of a periodic table they've isolated the first three rows of the periodic table so this is not the entire table um but what what you can see here is where we left off we were talking about the first shell remember the first layer of cake I call it the second shell the third shell let's look at how the periodic table is arranged remember I said that hydrogen is at the top left of the table because it has one proton and by the way if this is an atom and it has one proton how many electrons should it have if it's an atom it should have one electron and that's what this is right here that's your single electron right that that yellow dot represents its lone electron and that makes sense right because hydrogen hydrogen has one proton so it should have one electron does that make sense and if we move over and scan to the right and we look at now helium helium should have two protons right because it's the second element on the table and if it has two protons how many electrons would it have it should have two electrons and which shell do you think which shell should possess those two electrons well of course the most stable one right the the one with the least energy the first shell possesses the two electrons does that make sense now let me ask you this if I had three protons let's say I'm lithium okay lithium has three protons how many electrons would it have yeah it should have three electrons but let me ask you this two of those electrons occupy the first shell which only has one orbital where would the third electron live would it could it join the first shell no remember the first shell can only house two electrons so that electron that third electron needs to kick out to the next Shell it needs to exist in the second shell so you have one electron living in the outermost shell and what was the name for the outermost shell yeah that's right it was called the valence shell good so let me ask you this I'm going to quiz you how many valence electrons does a lithium have that's right it just has one so lithium has one valence electron now let me quiz you again how many total electrons does lithium have that's right lithium has three total electrons so just be aware that asking total electrons is a completely different question than asking valence electrons now let's look at beryllium beryllium has four protons so you guessed it it has four electrons two of the electrons exist in the first shell and two electrons in the second shell boron has five protons and five electrons carbon has six protons and six electrons and again let me ask you this how many electrons can exist in this second shell do you remember from my when I was at the board that's right the second shell can house more electrons what right do you remember why it's because the second shell the second layer it doesn't just have one orbital it has four orbitals remember that a spherical orbital and three dumbbell shape orbitals and let me just show you real quick what it looks like the first shell possesses just this spherical orbital called the 1s orbital but that and so it can only house two electrons that live in this orbital this spherical orbital and then when you kick it out to the second shell the second shell has four total total orbitals one that looks like a bigger sphere it's called the 2s orbital and then three that look like dumbbells now watch my mouse this is one of the dumbbell shapes it kind of looks like the infinity symbol this is the z p orbital okay this is a p orbital Z and then here's another p orbital called Y and then here's the third one in green this is the XP orbital you see so so an atom with two shells would look something like this at the bottom it would have one orbital in the first shell but it has three orbitals in the second shell and keep in mind each orbital houses two electrons so again how many electrons can exist in the second shell which houses four orbitals which has four orbitals that's right eight that's why you have the second shell with one two three four five six seven eight electrons see neon neon has one two three four five six seven eight valence electrons can can we add any more valence electrons to to Neon could neon accept another electron the answer is not in the second valence shell so when we kick it out to sodium when we look at sodium which has 11 protons 11 protons its 11th electron needs to start a third shell okay now we've got one electron in the third shell and again how many how many orbitals are in the third shell there are four more orbitals in the third shell because the the third shell possesses another large spherical orbital and and three more Pi p p orbitals so how many total electrons can exist in the third shell that's right eight more electrons so argon argon has eight valence electrons do you see what happened here notice this final column look at this last column here on the right okay this last column these are known as the noble gases and the reason they're known as noble gases is because they have filled their valence shell with electrons if I'm talking about the first shell for helium how many electrons fill that shell two right so helium has a full valence shell if we look at neon how many electrons fill the second shell eight right so neon has a full valence shell argon has a full valence shell so the final column on the periodic table of elements are all of the elements with a full valence shell that's why they're on the final column on the periodic table of elements so now we know what now we know about the periodic table of elements and its layout don't we we know why hydrogen is the first element it's because it has one proton we know why helium is the second one because it has two protons further we know why helium is pushed out to the last column it's because it has a full valence shell and now don't we understand the columns too The Columns this column has full valence shell what's the First Column have in common let me ask you this what's the what do the columns have in common can you tell me look at this closely you see this is a column what do these three elements hydrogen lithium and sodium have in common with one another what do the columns have in common with one another yes that's right they all have one valence electron and that's why they're in the same column look at this hydrogen has one valence electron lithium has one valence electron sodium has one valence electron and that's why they're in the First Column beryllium is in the second column along with magnesium because they both have two valence electrons Boron and aluminum have three carbon and silicone have four you see how how the columns work now we know what the columns mean and let me tell you this this is really exciting the the elements that have the same number of valence electrons tend to behave in a similar way in chemical reactions Isn't that cool it's the valence electrons that really dictate an element's chemical reactivity these these noble gases they do not react in chemical reactions they don't participate in chemical reactions and that's because they have failed their valence shell with electrons they are noble gases um similarly hydrogen lithium and sodium tend to behave in a similar fashion because they all have one valence electron carbon and silicon they behave in a similar fashion because they have the same number of valence electrons fluorine and chlorine they behave the same in chemical reactions or in a very similar way anyway in chemical reactions because they have the same number of valence electrons isn't that neat so it's the valence electron complement that gives chemical properties to different atoms isn't that neat now what and what did the rows represent let's take a look here real quick what do the rows represent that's right shells hydrogen and helium only have electrons in the first shell they don't have a second shell and that's why they're in the First Co they're in the first row now lithium through neon these are elements where the valence shell is the second shell so this is the second shell and then sodium through argon these are the elements with the electrons in the third shell and their their third shell is their valence shell and that's that's what we're talking about isn't that neat so now you know exactly why the periodic table is laid out in this fashion you understand that different atomic numbers dictate different element names and identities you understand what the columns represent what the rows represent and why noble gases do not react isn't that neat now let me share something else with you that's really interesting remember I told you that the noble gases and this final column of the periodic table like helium neon argon and so on these are inert gases which means that they do not react so if you're trying to run a chemistry lab and you're trying to make cool reactions occur well these elements are not your go-to elements for having fun though they are kind of neat for making neon signs you know you the neon signs that you see in cities but other than that there's really not a lot of cool chemistry going on with these noble gases because they don't react with one another and they don't react with any of the other elements so this is why chemistry happens this is really interesting the reason why chemicals react with one another and the reason why chemistry occurs and chemical reactions are happening all around us every day is because these other elements who are not the noble gases these other elements want to feel what it's like to be a noble gas they seek to fill their outermost shell with electrons that's their mission their mission is to fill their valence shell with electrons and get to a more stable State like that of the noble gases Isn't that cool so the entire reason why chemical bonds form and chemical reactions occur is because the other elements are trying desperately to fill their outermost shells and feel what it's like to be Noble to be a noble inner gas right so they want to fill their valence shell with electrons and that's why oxygen will react with hydrogen to make water right this is why the different elements react with one another this is why chemistry goes this is why chemistry works this is why chemistry is interesting and there are chemical reactions happening all around us it's because chemical bonds are forming because these different elements they need to work together in order to either steal from one another steal electrons give away electrons share electrons all in an effort to fill their outermost shell and the reason why chemical bonds form the reason why two elements two atoms come together to form a bond is because of that reason it's because they're trying to fill their outermost shell and the way to do that is what it's to share electrons steel electrons or giveaway electrons so that's the next concept after we check out what Gizmo and Wicket are up to and I'm sure it's not that good we'll come back and we'll talk about chemical bonds and just how these elements work out their differences in order to fill their outermost shells [Music] so let me give you an example of how atoms can share electrons to fill their outermost shell and become more stable and feel like those noble gases so let me give you the most basic example do you remember your friend hydrogen H for hydrogen remember it only had one proton so how many valence electrons does hydrogen have well it only has one electron right now let me ask you this this hydrogen have a full valence shell no it can house one more electron in that shell does that make sense remember because hydrogen has one valence electron that means it's in the first shell the first shell can house two electrons so hydrogen wants one more electron to fill its outermost shell now so what do you think would happen if another hydrogen were to come along right another hydrogen with its one electron were to come along and they meet could they work out some kind of deal in order to build their outermost shells and that is that's sharing right look if this hydrogen shares its electron with this hydrogen they're each sharing with each other this is known as Lewis Dot Structure format of writing by the way then guess what by sharing with one another they each feel like they have access to two electrons they feel like they've filled their outermost shell right so look this hydrogen thinks it has two electrons this hydrogen thinks it has two electrons it they filled their outermost shell it's like if you share a car with your brother your brother has a car you have a car you're saying you're sharing each other's cars what do you tell all your friends right what do you tell your friends you have two cars but do you technically have two cars no because one's technically your brothers but he's sharing with you it's just kind of a fun little analogy but again by sharing those electrons these hydrogens believe they have access to two electrons and so they believe they are they have a full valence shell so this is a much more stable molecule this is now formed in into a molecule called H2 or hydrogen gas okay and another way of drawing this is with a line showing that sharing okay now this is this is sharing right once you're sharing electrons you need to hang out with one another that's called a bond right and any time you form a bond by sharing the electrons that's known as a covalent bond so there is a single covalent bond between two hydrogens holding them together as hydrogen gas now what if different elements come into contact let's give you an example of that what about oxygen do you remember how many valence electrons oxygen had that's right it had six valence electrons so let's draw those valence electrons one two three four five six okay six valence electrons for oxygen so it's oxygen satisfied has it filled its outermost shell has it filled that second shell with electrons or could it have more it could have more right because remember the second shell can house eight electrons so two more oxygen is looking for two more electrons now what happens if a hydrogen floats by a hydrogen with its single electron could they maybe work out some kind of deal well yeah look the oxygen could say hey Ox uh the oxygen could say hey hydrogen I'll share this electron if you share yours right and now they're sharing the oxygen is sharing an electron with hydrogen and hydrogen is sharing an electron with oxygen so there's a covalent bond formed between the oxygen and the hydrogen now oxygen hydrogen is satisfied isn't it hydrogen satisfied hydrogen's happy it has access to two electrons hydrogen's great but what about oxygen is oxygen foliate let's count oxygen believes it has access to one two three four five six seven electrons close but no cigar right so one more hydrogen floats along here's another hydrogen can they work out a deal yep oxygen will share this one hydrogen will share that one and they are sharing Now isn't everyone happy let's take a look this hydrogen has two electrons this hydrogen has two electrons and the oxygen sees how many one two three four five six seven eight and it takes eight electrons to fill the second shell so everyone is happy everyone in this partnership has access to a full valence complement a full valence shell isn't that awesome so this is the most stable that an oxygen and two hydrogens can become it's it forms a molecule called what what do you think this is called there's two h's and an o H2O this is water right water wonderful and that is why water forms oxygen and hydrogen form covalent bonds single covalent bonds and again you could draw this as o line h line H every time you see a single line like that it's a single covalent bond and water is a molecule with two hydrogens and an oxygen and now you know why water forms all right let me show you another option that oxygen has that's very common remember oxygen has six valence electrons we had mentioned that just in a previous example oxygen has six valence electrons it wishes it had two more electrons right now what do you think happens if another oxygen Moses along here so you have another oxygen right next to it with its six valence electrons each oxygen has six valence electrons you see what I'm saying you think they can sort out some kind of deal well let's start sharing this oxygen says I'll share this one and this oxygen wants to share that one so now each oxygen is sharing an electron with with the other oxygen does that make sense now let's see if this is satisfactory have the oxygens fill their valence shell so this oxygen sees one two three four five six seven okay so so again this is a close situation but we could do better right if if if this oxygen were to share another electron and this oxygen were to share a second electron now now what do we have have the have the oxygens filled their valence shell let's see oxygen sees one two three four five six seven eight this oxygen sees one two three four five six seven eight they filled their outermost shell but did you see what happened each oxygen had to share two of its electrons with the other partner and so this forms a molecule isn't this sharing too that's sharing isn't it so these are covalent bonds so this is known as oxygen gas or O2 however however because each of these oxygens had to share two of their electrons with the with this with the single partner this is known as a double covalent bond and and you would draw it with two lines does that make sense you would need to put two lines between the oxygens to show that there are two covalent bonds you see it's a double covalent bond um for every electron you share with a partner that's a single covalent bond so remember hydrogen shared one electron with hydrogen that's a single covalent bond for hydrogen gas but here oxygen is sharing two electrons with another oxygen so that's actually a double covalent bond now you might be asking is there such thing as a triple covalent bond Dr D and there is let me show you okay so let me show you this nitrogen nitrogen has five valence electrons okay five valence electrons so how many more would nitrogen need in order to fill its second shell its outermost shell that's right it needs three more valence electrons to fill that shell because remember it can have eight electrons in this shell so here we go another nitrogen comes along and it has five valence electrons can they sort out something maybe a little sharing deal yes this nitrogen says I'll share this electron this nitrogen shares one electron now they're each sharing one now let's see if we filled this shell one two three four five six okay uh not yet okay we need to share some more this nitrogen shares with this okay each one shares another electron now each nitrogen sees one two three four five six seven closer but still not good enough in this case this nitrogen will share a third electron with this nitrogen we'll share a third one as well now look at this now are they satisfied now they're satisfied now each nitrogen has access to one two three four five six seven eight now the nitrogens have filled their outermost Shell by each nitrogen shared three electrons with its partner so this would be an N with three bonds to another n and that's known as a triple covalent bond forming a gas called N2 also known as nitrogen gas now as I mentioned at the board atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms when this happens these interactions usually result in atoms staying close together held together by attractions called chemical bonds and the type of chemical bond we just covered on the board is a covalent bond where there's sharing of a pair of valence electrons by two atoms so remember on the board I showed you two hydrogens each hydrogen with one valence electron they came together they shared those valence electrons and we formed hydrogen gas so this is a type of covalent bond a single covalent bond and when a covalent bond forms that forms a molecule a molecule consists of two or more atoms held together by covalent bonds again a single covalent bond is the sharing of just one pair of valence electrons a double covalent bond is the sharing of two pairs of valence electrons and do you remember the notation we used to represent atoms and bonding there is a line right a single covalent bond is denoted with a single line however a double covalent bond is denoted with a double line and the molecular formula is written out like this H2 or O2 or H2O and here we can see some of the molecules that I had drawn on the board this is a hydrogen molecule drawn one two three four different ways and oxygen drawn for different ways and remember oxygen has a double covalent bond a water molecule drawn for different ways and methane drawn four different ways this here is known as the electron distribution diagram when you see molecules drawn this way this one here is the Lewis Dot Structure this one is called the structural formula and lastly this is the space filling model it shows you how much space those atoms take up and as part of the molecule and do you remember that the valence electrons are the electrons in the outer shell covalent bonds can form between atoms of the same element or atoms of different elements I showed you that hydrogens can bond together to form H2 or they can bond to other elements as well Like Oxygen hydrogen and an oxygen combined a compound is a combination of two or more different elements now when molecules form we need to understand something that can happen when this sharing occurs in covalent bonds and to do so we need to understand this concept of electronegativity so let me show you this atoms in a molecule attract electrons to varying degrees this means that different atoms attract electrons to different degrees this is known as electronegativity electronegativity is an atom's attraction for the electrons in a covalent bond some atoms are more attracted to the electrons they pulled the electrons closer to them and some are less so and so they do not pull electrons closer to them the more electronegative an atom is the more strongly it pulls shared electrons towards itself and look at the periodic table of elements right here at the bottom take a look at this what I'm what I want to show you is that electronegativity an atom's pull or a affinity for electrons increases as we look to the right of the periodic table so the the elements on the right side of the periodic table have more electronegativity than on the left side and as we look up on the periodic table so the most electronegative of the elements are right here at the top right of the periodic table and the least electronegative of the elements are down here at the bottom left of the periodic table one thing about electronegativity that I need you to know it's a very important exception to the rule and remember the rule that I mentioned was that the electronegativity of atoms increases as you look to the right and up on the periodic table of elements electronegativity increases but there's one exception that you need to know and that's this on the periodic table hydrogen hydrogen should actually be right on top of carbon okay so it would look a little something like this all right so why is that I want to tell you something hydrogen and carbon have roughly the same electronegativity so that's why I I'm telling you this exception you should know you should remember that hydrogen and carbon have roughly the same electronegativity and that means that if they're going to share electrons they're going to share electrons evenly okay um so carbon and hydrogen have roughly the same electronegativity and students ask why why Dr D why why does hydrogen and carbon have the same greediness for electrons why why would hydrogen and carbon have the same electronegativity why isn't hydrogen like lithium or sodium which is on the left side of the periodic table well let me explain that real quick hydrogen hydrogen has how many valence electrons do you remember let's draw this out hydrogen hydrogen has how many valence electrons one right one how many does it wish it had to fill its valence shell another one right so so hydrogen wishes it had one more to fill its shell so would you agree that hydrogen was half full of electrons yep okay now now see where I'm getting with this carbon has how many valence electrons do you remember carbon has four carbon has four valence electrons and it exists on the second row of the periodic table so it's its valence shell is the second shell how many more electrons does carbon require to fill that shell four right so four more it has four and it wants four more to fill its shell so would you agree that carbon is also half full of electrons in its valence shell you see where I'm getting hydrogen is half full it has one electron it wants to carbon is half full it has four valence electrons and it wants eight the elements that are half full are going to be equally greedy for electrons whereas hydrogen if it's way over if hydrogen is way over near um sodium do you see the problem here sodium has one valence electron but how many more does sodium want or lithium okay lithium with this one valence electron DC why hydrogen would not have the same greediness as lithium or sodium lithium has one valence electron just like sodium does but lithium needs seven more right so it's not going to act the same sodium has one valence electron it one seven more okay so this is what we're talking about hydrogen is half full so it's going to be as greedy as carbon which is half full and silicon which is half full isn't that neat so this is why hydrogen has the same electronegativity roughly as carbon so now let's talk about types of covalent bonds there are different aspects of how covalent bonds behave between different atoms and I just want to share share this concept with you what again if hydrogen hydrogen has one valence electron and another hydrogen has one valence electron remember what they do when they come into contact they share so this hydrogen shares an electron with this one and now a single covalent bond has formed so now let me ask you this hydrogen is sharing with hydrogen so are those electrons going to be shared evenly among the two remember the electrons are not stuck in the middle like this I know this is a Lewis Dot Structure and it looks like the electrons are stuck in the middle but they're not the electrons can the electrons can be anywhere in those orbitals right so so both electrons could be here one second then the next split second one electron might be over here and one electron is over there the next Split Second both electrons are over here so the electrons are pretty much anywhere in this Cloud but let me ask you this if these two hydrogens are sharing these two electrons and they're both equally greedy for the electrons are those electrons going to spend too much time with this hydrogen probably not are they going to spend too much time with that hydrogen probably not so the electrons are going to be shared evenly so imagine this again remember my example with sharing a car with your brother if I have a car and my brother has a car and we're sharing the cars if we're both equally needy of a car and equally greedy for the car we're both going to spend the same amount of time with the cars does that make sense okay so this means that the electrons are shared evenly anytime anytime electrons are shared with the same element two of the same element they're going to be shared evenly so if electrons are shared by two hydrogens the electrons are shared evenly if electrons are shared by remember this one two oxygens the electrons are going to be shared evenly if electrons are shared by two do you remember this one nitrogens with their triple covalent bond the electrons are going to be shared evenly again because oxygen is not greedier than oxygen anytime you have sharing of electrons between two equally greedy Partners the the sharing is going to be even and what does that mean the sharing is going to be even again that means that the electron spend these electrons that are being shared spend just as much time with this oxygen as they do with this oxygen neither the oxygens is hogging those electrons okay and um so by the way when this happens when when electrons are being shared evenly to form a covalent bond those aren't just covalent bonds those are known as nonpolar covalent bonds so hydrogen gas oxygen gas and nitrogen gas are forming not just single double and triple covalent bonds respectively but they are nonpolar covalent bonds and that's why now let me show you a little bit different scenario remember your oxygen friend oxygen has six valence electrons and do you remember that's why it pairs up with two hydrogens remember to form water we talked about this so these are all sharing so there's a there's a covalent bond between the oxygen and this hydrogen and there's another covalent bond between the oxygen and this hydrogen right but now let me ask you this do you remember how electronegativity works the electric electronegativity is greater for atoms that are to the right and up on the periodic table of elements and do you remember even by putting even if you put hydrogen on top of carbon oxygen still to the right which means it's more electronegative isn't it then do you remember what electronegative means electronegative means greedy for electrons it technically means the Affinity the Affinity of an atom for electrons so oxygen has a greater affinity for electrons oxygen is greedier for electrons than hydrogen why again because it's to the right on the periodic table it is greedier so do you think if oxygen is greedier than these hydrogens that the sharing is going to be even Steven right so if my brother is much greater than me and I'm sharing a car with him am I going to have equal access to that car or is he going to kind of hog that car and the car is going to spend too much time with him you see what I'm saying so look at this the electrons here you see the electrons are supposed to spend the same amount of time with the oxygen as they do the hydrogen that's what sharing means right but if the if the if the if the oxygen is kind of like this electron magnet like this electron vacuum cleaner then the electrons are spending way too much time with this oxygen and not so much time with this hydrogen okay and you might be wondering well so what what why do we care if the electrons are spending more time with the oxygen than the hydrogen well it's still sharing but it's an uneven sharing and that's what you need to understand the sharing is uneven the electrons are spending way more of their time with the height with the oxygen than the hydrogens now do you remember what keeps these atoms and these molecules neutral remember we talked about neutral when you have a proton and an electron close by that keeps everything neutral right can't if you have a plus and a minus that's when you have neutrality right so these hydrogens have a proton right but what do you think happens when it's negative charge drifts away or it's spending too much time with that oxygen well think about it what charge are these electrons what charge do electrons have they have a negative charge and that negative charge is bending too much time where that negative charge of spending too much time with this oxygen that means that this oxygen starts becoming partially negatively charged this is not an s this is the symbol for the Greek letter D lowercase D for Delta and this signifies that the oxygen is partially negatively charged and that leaves the hydrogens partially what they are partially positively charged all right did you see what happened for these charges to not appear you would have to be sharing the electrons evenly but because oxygen is greedier and it's like hogging those electrons a little too much that negative charge is kind of culminating here it's manifesting here on the oxygen as a partial negative charge and the hydrogens are left with a partial positive charge and this is known as not just a covalent bond but because it's uneven and it's and it's forming these poles this is known as a polar covalent bond and yes this is why some molecules are called polar molecules you ever heard that water is a polar molecule well now you know why now are you ready for a tricky example here all right let's let's think about this one if we have carbon and do you remember that carbon has four valence electrons how many electrons does carbon need to fill its shell carbon needs four more electrons to fill its shell right it has remember it's half full it has four electrons it needs four more so what do you think happens if a hydrogen floats by yes that's right they can share is carbon happy no not yet so what if another hydrogen were to share it's more happy another hydrogen could share and a fourth hydrogen share so look at this this is a carbon filling its outermost Shell by sharing with four hydrogens you see carbon can form four bonds because it needs four more electrons that's why carbon can make four bonds right so this is a molecule called methane and methane is c h four okay now let me ask you this are these covalent bonds between the carbon and the hydrogen yes they are covalent bonds but now this is the part which requires you to recall are these polar covalent bonds or nonpolar covalent bonds exactly they are non-polar covalent bonds why because of that very important exception I needed you to remember that carbon and hydrogen have roughly the same electronegativity they are equally greedy for electrons so even though they don't have the same identity carbon and hydrogen aren't the same thing you know because I told you that exception because I told you carbon and hydrogen have the same electronegativity they do share the electrons evenly okay so the correct answer is carbon and hydrogen forming methane CH4 these develop single nonpolar covalent bonds okay and if that's true let me ask you this if there if everyone's sharing the electrons evenly would there be any charges forming like partial positive or partial negative charges anywhere if you're sharing evenly do these partial charges pop up anywhere no there shouldn't be any partial charges popping up because for every proton there's a electron nearby just like there should be and so no charges pop up and this is a nonpolar molecule so if you ever heard of nonpolar molecules this is a nonpolar molecule and that's why now let me tell you something fascinating when in the next chapter we're going to learn about biomolecules we're going to learn about fats and oils fats and oils are typically nonpolar isn't that neat fats and oils are nonpolar and did you know that fats and oils and lipids in general they are mainly made of carbon and hydrogen do you see why fats and oils and other lipids are nonpolar okay uh well that's because fats oils and lipids are mainly giant molecules made up of carbons and hydrogens and because they're mainly carbon and hydrogen and carbon and hydrogen form nonpolar bonds that makes those fats and oils nonpolar so again nonpolar covalent bonds are covalent bonds where atoms share the electrons evenly no partial charges form and this results in a nonpolar molecule and in polar covalent bonds one atom is more greedy more electronegative and the atoms do not share the electrons equally this results in partial charges unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule remember for example in water oxygen was left with a partial negative charge because oxygen is more electronegative while the hydrogens were left with a partial positive charge because they are less electronegative so here you can see what I'm talking about here's the the lowercase D Greek letter I was telling you about Delta here you have a partial negative charge on the oxygen and a partial positive charge on each of the hydrogen atoms in this water molecule and these yellow arrows are showing you this uneven distribution of electrons oxygen is hogging those electrons the electrons are being drawn closer to oxygen spending too much time with oxygen then this is what's responsible for this polarity and that's why water is known as a polar molecule because it develops these polar ends now let me ask you this what do you think happens when a water molecule and remember a water molecule is a oxygen and two hydrogens right and do you remember the partial charges that form on that water molecule due to the polar covalent bonds the hydrogens have a partial what charge a partial positive charge and the oxygen has a partial negative charge right exactly so what do you think happens if another water molecule Moses by okay remember here's here's a water molecule here's a different water molecule the water molecules have partially positive hydrogens and a partially negative oxygen do you think this water molecule can interact with that water molecule yeah they can interact actually and let me tell you this at the molecular level Opposites Attract you need to know that at the molecular level Opposites Attract so positively charged things like to stick to negatively charged things positive and negatives like to come together so what part of the water molecule is negative that's right the oxygen and one what part of this water molecule is positive that's right the hydrogen so do you think this hydrogen which is partially positively charged might want to stick to and interact with this oxygen that is partially negatively charged that's exactly right they will kind of stick to one another just it's a loose stickiness it doesn't make it a bigger molecule by the way so it's a type of bond but it doesn't make this like a big molecule it just me it just means this molecule of water likes to stick a little bit to that water molecule okay and why do they like to stick together well because this partially positive charge wants to hang out with this partially negatively charged and that stickiness is denoted as a bunch of dots right so when you see little dots like this when you see little dots like I drew there that means there's a little bit of stickiness between oppositely charged partial charges right and that's known as a hydrogen Bond okay a hydrogen bond isn't that neat the hydrogen bond formed between the two water molecules so it's like a little it's a very weak Bond hydrogen bonds are weak interactions now let me ask you this would two water molecules want to interact in this fashion with the oxygens pointing to one another let me ask you that would two water molecules want to have their partially negatively charged oxygens stick to one another no they wouldn't the the oxygens would not want to interact and hydrogens hydrogens would not want to interact either right so look at the hydrogens would not want to interact the oxygens would not want to interact between different water molecules so if this were to happen this water molecule would flip around right it would flip around and put its hydrogen here okay does that make sense because it's hydrogens partially positive the oxygen is partially negative and how do we denote a a hydrogen bond that's right little dots right so hydrogen bonds and and this this water molecule would flip around too right this water molecule would flip around where the oxygens over here and that partially negatively charged oxygen would stick to the partially positively charged hydrogen and so water molecules water molecules because they are polar they will stick to one another so polar molecules can stick to other polar molecules and that's because of those partial charges and the partially positive charge of one polar molecule will like will like to stick to the partially negatively charged of another water molecule does that make sense and that stickiness of water molecules to one another has a name when water molecules stick to one another by hydrogen bonding with one another this is called cohesion now what I've drawn for you here is the Lewis Dot Structure of ammonia nitrogen with three hydrogens NH3 ammonia now let me ask you this ammonia is it a polar molecule or a nonpolar molecule how would you figure that out well do you remember electronegativity right is nitrogen to the right of hydrogen where I told you hydrogen should be hydrogen should be right on top of carbon nitrogen is to its right that means that nitrogen is actually more electronegative than hydrogen nitrogen is greedier so where do those electrons spend more of their time the electrons are being hogged by nitrogen and do you remember electrons have a negative charge so the negative charge is being drawn to the nitrogen the nitrogen develops what a partial a partial negative charge leaving the hydrogens with a partial positive charge okay so again let me ask you this are these polar covalent bonds between the hydrogens and the nitrogen or these nonpolar covalent bonds these are polar covalent bonds because of the uneven sharing because nitrogen is more electronegative than hydrogens and so this is a polar molecule ammonia ammonia is a polar molecule so what does that mean for you why why why do we care well let me ask you this what do you think happens if a water molecule comes along let me ask you this think about this water remember water is polar the hydrogens have a partial positive charge and the oxygen has a partial negative charge can that water molecule stick is it can it stick to ammonia the answer is yes right remember Opposites Attract this nitrogen this nitrogen is partially negatively charged this hydrogen on the water is partially positively charged can they stick to one another yes and what are these that I just drew what are these little dots that denotes what a hydrogen Bond that's right hydrogen bonds can form between the hydrogen of water and the nitrogen of ammonia and what if another water molecule was over here what part of that water molecule would interact with the hydrogens of ammonia that's right it would be the oxygen The partially negatively charged oxygen remember the oxygen is partially negatively charged on this ammonia the hydrogen is partially positively charged would there be a stickiness and how do you denote the stickiness that's right a series of dots showing a hydrogen bond so do you agree that water can stick to polar molecules such as ammonia well guess what that's important to understand you know why because ammonia dissolves in water have you ever heard that water is a good solvent have you ever heard that term water is a good solvent water dissolves a number of things a solvent is a chemical in which you dissolve things into right and water is a great solvent it you can use water to dissolve lots of stuff and you could dissolve ammonia in water what do you think Windex is for instance all right so let me tell you something anything that water can stick to and make hydrogen bonds with can be dissolved in water okay water can dissolve any substance that's polar or charged isn't that neat for example sodium ion sodium ion n a plus sodium ion do you think water could dissolve sodium ion na Plus and how what part of the water molecule would dissolve sodium ion that's right it's oxygens right The partially negatively charged oxygen of water would form a hydrogen bond with the Sodium so water can dissolve sodium what about chloride chloride is CL minus can water dissolve chloride yes and what part of the water would stick to the negatively charged chloride that's right the hydrogen so if I had a if I had a water molecule here The partially positively charged hydrogen would hydrogen bond with the chloride isn't that neat so do you think chloride can be dissolved in water the answer is yes as a rule of thumb you know what I like to say if water can stick to a molecule or an ion water can dissolve that molecule or that ion so what kinds of What kinds of substances can be dissolved in water again water can dissolve anything that's polar that means it has partial charges or charged that means ions either cations positively charged ions like sodium or anions negatively charged ions like chloride isn't that neat so this is why water is not a universal solvent you ever you ever tried to dissolve oil in water what happens when you try to dissolve oil in water that's right they separate water can't dissolve oil they separate out right oil and water separate so let me ask you this before I show you before I show you what property do you think oil has that doesn't allow water to stick to it that's right it's nonpolar right oil is nonpolar fats are nonpolar waxes are nonpolar these types of substances don't dissolve in water because they are nonpolar so I'm going to show you an example what was this molecule do you remember we have a carbon remember carbon has four valence electrons but here it's sharing electrons with four hydrogens what was this molecule called this is C h four also known as that's right methane methane and what did we know about the properties the chemical properties of of methane is it a polar molecule or a nonpolar molecule and why remember that's right it's a nonpolar molecule why because there are no partial charges anywhere why because hydrogen and carbon are equal in electronegativity and that means that the electrons in the covalent bonds are are shared evenly right so this is a nonpolar molecule there's no partial charges or full charges anywhere now let me ask you this if a water molecule were to Mosey by okay see this water molecule right here remember oxygen has what charge by the way let me tell you something if you say oxygen is negatively charged and just put a minus there and you say the hydrogens are positively charged and just put a plus there that's actually wrong is that a full negative charge on that oxygen no it's a partial remember you have to put the Greek letter d right and is that a full plus charge on those hydrogens or a partial plus charge it's a partial plus charge so Greek letter D on those anyway water is a polar molecule it has partial charges due to the polar covalent bonds methane is a nonpolar molecule it does not have partial charges because it has nonpolar covalent bonds so let me ask you this all right think about this what part of water would stick to methane go ahead and think about it with the negatively The partially negatively charged oxygen stick to any part of methane no because that that that oxygen wants to stick to anything that's positively charged with the partially positively charged hydrogens want to stick to methane anywhere nope because those would want to stick to something that's in some way negatively charged right so let me ask you this what part of water would stick to methane that's a trick question right no part no part of water would want to stick to methane water only knows how to stick to you if you have a charge if you have a plus charge of some kind be it a partial plus charge or a full plus charge if you have a negative charge of some kind be it a partial negative charge or a partial or a real full on negative chart you see what I'm saying water only knows how to stick to molecules if they have some kind of charge and remember if water can stick to you water can dissolve you right can water stick to methane no so do you think water could dissolve methane no okay now you get it and guess what oils fats and waxes have in common with methane do oils waxes and fats mix with water no and why is that well because we're going to learn about this oils fats waxes all the lipids right the most of the lipids most of the lipids they are made up of almost entirely C and H which means they're nonpolar there's they're making nonpolar covalent bonds so fats oils waxes they're these big molecules that are just carbons and hydrogens there's no partial charges anywhere there's no fault charges anywhere water can stick to you and if water can't stick to you and water separates out right and oil water oil and water separate do you see now why oil separates from water now you understand at a basic fundamental molecular level You Now understand deeply why oil and water don't mix isn't that neat so these hydrogen bonds that I spoke of these are an example of weak chemical interactions hydrogen bonds form when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom so I showed you some examples with water forming the hydrogen bond with ammonia and remember how you denote a hydrogen bond with little dots and remember that water can also hydrogen bond with itself a process known as cohesion and also water can hydrogen bond with pretty much any other ions right and positively charged negatively charged anything with a charge can form bonds with these weak bonds with water now if electrons are not evenly distributed in a molecule they may accumulate by chance in one part of a molecule and we can see that this can cause weak interactions called Van Der waals interactions interactions between molecules that are close together as a result of these charges these partial charges and here you can see a gecko's toe is designed to take advantage of these weak interactions these Van Der waals interactions so now let me show you a different scenario with a different outcome with a different Arrangement all right this here is sodium atom n a and remember sodium is in the first column on the periodic table so it has one valence electron chlorine is in the second last column so it has seven valence electrons chlorine just needs one more to fill its shell however let me ask you this how many more does sodium need to fill its shell it's in the second row right so sodium technically needs seven more oh goodness right seven more okay think about that for a second sodium needs seven more and chlorine only needs one more now what do you think would happen let's let's let's think about this I asked my students to think about this right so imagine if sodium come comes close right sodium comes close I ask okay could they share could sodium share its one valence electron with chlorine and let's just say let's just say they did share now is chlorine satisfied has chlorine filled its outermost shell and the answer is one two three four five six seven eight chlorine is good chlorine has filled its outermost shell but now let's look at this sodium over here um sodium now has access to two so remember sodium needs six more and it's it's not going to find six more electrons that would mean it would it would need to partner with six friends on this side and that's just not gonna happen so so this so there's no covalent bond that forms between sodium and chlorine because chlorine would be happy but sodium would not right and that's why a covalent bond would not form between sodium and chlorine sodium just would not have filled its shell and it wouldn't be able to fill its shell so this would not be a stable molecule does that make sense but let me ask you this let me ask you this what would happen if sodium with which has one valence electron what if sodium just said hey why don't you just take take my electron take it it's yours think about that for a second so not sharing this is not sharing sodium literally gives its electron to chlorine so sodium gives its valence electron to chlorine so let me ask you this what happened to the third shell of sodium that's right it's gone the third shell disappears and now sodium has a full second shell isn't that neat the third shell disappears you drop down to the second shell and the second shell of sodium is full so by giving its one valence electron to chlorine it obliterated that valence shell it dropped down to the next smaller shell it dropped down to the second shell which is already full so sodium it cheated it didn't have to find seven other electrons by giving away that one it filled its outermost shell that was the easiest way for sodium to fill its outermost shell was to just give away that electron does that make sense but there's consequences right when you give away an electron what happens do you remember the definition of an atom an atom has the same number of protons and electrons okay so the only reason sodium was an atom and sodium had no charge is because it had the same number of protons and electrons does that make sense but what did it just do it just gave chlorine an electron so now doesn't it have one more proton than it does electrons it just gave away a negative charge it just gave away an electron so what charge is left over it it has one positive charge sodium has one positive charge not a partial positive charge a full-on positive charge so sodium has a positive charge and look at chlorine chlorine just gained an electron it outright stole that electron does that make sense so chlorine has what charge chlorine has now an extra negative charge so chlorine becomes what's called chloride which is a negatively charged right a negative charge entity so now let me ask you this just to test your understanding is sodium still called an atom no because remember by definition atoms have an equal number of protons and electrons so sodium is now called an ion and chlorine is now called an ion as well so this is not called sodium atom anymore because it does not have an equal number of electrons and protons it is called sodium ion and this is called chloride which is an ion as well or chlorine ion which is known as chloride and by the way a positively charged ion is known as a cation and a negatively charged ion is known as an anion one way I tell my students that they can remember that is that you know how I love my cat's Gizmo and wicked right everyone knows that well I always remember that cats are a positive thing right I even had a student tell me that pause you know the cats have paws so cats are a positive thing so cations are positive charges anyway it's a silly mnemonic device a silly trick to remembering the that you know positive charges are cations now let's get back to this so sodium cation and chloride anion form now what do you know about opposite charges what did what did I tell you about opposite charges opposite charges attract in chemistry in chemistry opposite charges attract so sodium is positively charged chloride is negatively charged and guess what simply because they have opposite charges they snap together all right they snap right together and that is formed and ionic Bond so is this a covalent bond no way there's no sharing going on covalent bonds are about sharing this is an ionic bond because it's the bond that forms between oppositely charged ions before we move on from ionic bonds I want to introduce some important terminology that's going to help you so much when we're talking about cellular respiration in a later chapter and that is the terms oxidized and reduced I oxidation versus reduction okay so bear with me let me let me explain this do you remember how what happened when sodium atom met chlorine atom what happened did they share electrons no sodium lost its electron to chlorine right so sodium lost its electron chlorine gained an electron and here's what I want you to know the term for losing an electron is called oxidation the term for gaining an electron is called reduction so in the reaction that I told you about where sodium atom meets up with chlorine atom remember what happened sodium lost its electron and now has a positive charge chlorine gained an electron and so what scientists would say is that sodium was oxidized and chlorine was reduced gain of sorry loss of electrons is called oxidation loss of electrons is called oxidation gain of electrons is called reduction so in this chemical reaction sodium atom was oxidized to sodium cation while chlorine atom was reduced to Chloride the anion here are some more interesting facts about ionic bonds so ionic bonds form uh anytime column one on the periodic table of elements column one encounters the second last column right column one encounters the second last column this kind of ionic bond will form and those com those form compounds like salt this NaCl is known as salt table salt table salt and did you know that any time column one meets the second last column assault is formed and there's more than one salt so for example potassium chloride forms because potassiums in the first column with chloride being in the second last column and potassium chloride forms an ionic bond between the potassium cation and the chloride anion and that's assault also for example sodium fluoride can form sodium fluoride that's another salt or potassium fluoride is a salt so salts form as compounds right compounds between column one and the second last column here's another confusing fun fact about ionic bonds I was really confused when I went to college because I took a chemistry class and a biology class at the same time so concurrently in my chemistry class I learned from the textbook that ionic bonds are strong bonds but then I took my biology class and the textbook said that ionic bonds are weak so which is it are they strong or weak and which one's wrong which textbook was wrong well it turns out they were both right and I just want to explain to you why real quick because I was confused and I don't want you to be confused that's why the reason is because chemistry deals with actual elements as they are the the pure elements so if I have let's say sodium chloride let's focus on sodium chloride here if I have sodium chloride what is that that's salt salt and if I have dry sodium chloride and it's a salt have you ever seen those big salt crystals or those big salt licks you could have a you could you could even make salt into rocks right which are hard and they're like impossible to pry apart with your fingers right they're hard I mean you throw the salt across the room and damage the wall right that's tough those ionic bonds are holding the sodium cation the chloride anion together really hard right that's hard that's a strong bond but what happens when you put that same salt crystal into water this day a crystal for long no because remember the water molecules they can dissolve salt remember because water can stick to sodium cation with its partially negatively charged oxygens water can stick to Chloride anion with its partially positively charged hydrogens so water dissolves salt so is this big strong scary ionic bond all that tough when it comes to water no and biology is the study of Life correct and life is based on what fundamental unit a cell a cell and what are your cells full of your cells are mainly water right most of the weight of your cells is water so everything that's happening in your body is happening in the presence of water does that make sense so ionic bonds are weak in the presence of water because water dissolves them okay but ionic bonds are strong in the absence of water like a salt crystal so this is why your Chemistry textbook may say that ionic bonds are strong while your biology text would say they're weak all right now to finish off the slides for this chapter and wrap it up remember we talked about ionic bonds where atoms sometimes strip electrons from their bonding Partners kind of like how chlorine stripped an electron from sodium the two resulting oppositely charged entities are called ions the positively charged ion is called the cation negatively charged ion is called an anion and anions and cations attract each other forming an ionic bond sort of like the compound salt NaCl table salt and by the way I told you about oxidation and reduction I have a fun way to remember how oxidation and reduction work the terminology I think of is Leo says ger that helps me to remember believe it or not and I'll tell you why Leo would stand for loss of electron means oxidation ger you know it's like a lion would say ger ger is gain of electron means reduction so every time I think what did oxidation mean I just remember that Leo says gur so Lions say ger and that helps uh me to remember I hope that helps you too so here remember I showed you how sodium gets oxidized to sodium cation while chlorine becomes reduced to Chloride anion and the cation and anion stick together and form an ionic bond forming a compound known as sodium chloride NaCl table salt and remember compounds formed by ionic bonds are called ionic compounds or salts this happens anytime the first group or First Column meets the second last group or second last column salts remember are often found in nature as crystals if they're if they're dry they are in crystalline form if they are in aqueous environments they're dissolved NaCl itself is not a molecule the formula for an ionic compound indicates the ratio of elements in the crystal of the salt most salts are quite stable when dry but dissociate quite easily in water we touched on that as well so here you can see the crystalline structure of salt in a salt crystal and remember water can easily pick apart this Crystal and bring it into solution now the last concept of this chapter is chemical reactions which include the making and breaking of chemical bonds the starting molecules of a chemical reaction are called the reactants and the resulting molecules of a chemical reaction are called the products and so reactants appear on the left of the of the arrow so here hydrogen gas and oxygen gas are the reactants and then they react there's a chemical reaction where they rearrange their bonds to form water two water molecules so these are known as the products on the right side of the arrow and chemical equilibrium is reached when the forward and reverse reactions occur at the same rate so you would see an arrow going to the right and a narrow going to the left awesome well we did it we got to the end of the chapter I know this was a long chapter but I hope my explanations helped to shed light on chemistry for you and give you a better understanding of how these little atoms and elements work now let's move on to chapter three where we will discuss water and life Dr D Dr D [Music] ude actually