hi everyone in the last lecture we talked about electrons and specifically where those electrons are orbiting relative to the nucleus we talked about electron shells and the fact that in this class you only need to know about the first second and third electron shell and remember that the first cell holds a maximum of two electrons and you can see that helium has to so it has a full outer electron shell then the second shell remember holds a maximum of eight electrons and you can see that neon has eight so that would be a full auto electron shell for neon and then finally that third shell also holds a maximum of eight and you can see that argon has eight in the outer electron shell so it also has a full shell sorry I didn't mean to do that yet if you look at hydrogen lithium sodium they all have one electron in that outer electron shell and that is because we can sketch the electron configuration and and determine that but we can also look at the periodic table in the shortcut is looking at these Roman numerals over these eight columns so skipping the metals in the middle here column one being this column with hydrogen lithium sodium everything in column one has one valence electron meaning one electron in that outer electron shell everything in column two has two valence electrons so beryllium magnesium calcium and so on have two electrons in that valence shell skipping over here to the column with Boron and aluminum and gallium everything in this column has three electrons in the outer electron shell if you haven't had Roman numerals in a while you could think of these just as a normal number you could write a one two three four five six seven eight over these eight columns so column four with carbon carbon has four valence electrons nitrogen has five oxygen has six fluorine has seven and finally neon has eight obviously column eight helium only has two electrons total so it does not have eight in the outer shell but it does have a full valence shell and everything in column eight because it already has a full outer shell it doesn't need it to do anything to fill that shell which means it is inert everything in this column is inert meaning non-reactive already stable these atoms don't need to do anything to fill their outer electron shell it's already full the other seven columns though are going to have to do something to fill that outer electron shell and they're going to have a couple of options that we're going to talk about today but this is your shortcut to finding out how many valence electrons an atom has and we will come back to this in a few minutes okay next I want to go to my iPad and write some statements for you and start drawing some ways that atoms can either share electrons or gain or lose electrons to complete that outer shell so every atom must have a full valence shell to become stable and one way that atoms can achieve that is through sharing electrons when two atoms share electrons they form what's called a covalent bond this is the first of three types of chemical bond that we're going to talk about covalent bonds involve two atoms sharing electrons and two or more atoms bonded together is what we call a molecule So today we're going to be looking at molecules for the first time in this class what do I mean by sharing electrons well let's look at the periodic table again for a minute and you'll recall that hydrogen is atomic number one now today I am only interested in the number of electrons that each atom has because we're only talking about electrons today not protons and not neutrons atomic number one tells us that hydrogen has one electron and remember that electron would be located in the first electron shell which holds a maximum of two electrons and hydrogen has one so hydrogen is not stable it has one open spot that it needs to fill to become stable also remember hydrogen is in column one which also which tells us that hydrogen has one valence electron okay similarly let's look at Carbon carbon is atomic number six which means it has six electrons total and it's in column four so column four tells us that carbon has four valence electrons the other way we can know that carbon has four valence electrons is to sketch it so carbon I'm writing the the chemical symbol of the element in the nucleus this time so we can keep track of which atom we're talking about here so first electron shell holds two and obviously carbon has more than two so those other electrons are going to go in the second electron shell which holds a maximum of eight electrons and I'm going to draw a carbon's green so 1 2 3 4 5 6. I've drawn them that way specifically so I can show sharing of electrons with carbon and hydrogen so carbon has four valence electrons that means that carbon has four spots that need to get filled for carbon to become stable and one way that carbon can do this is to share electrons with another atom in fact it can share with that hydrogen so let's see what that would look like here's our carbon again two electrons in the first shell and then four in the next Shell so that's atomic number six column four so four valence electrons that means there are four spots out here that still need to get filled for carbon to become stable and one way it can do that is to share electrons with hydrogen if this hydrogen comes over and shares its electron with carbon in this spot now that hydrogen has a full outer shell and in that spot carbon has a full outer shell and what if three other hydrogens come along and do the same thing so this hydrogen is going to share an electron with carbon and carbon is going to share an electron with that hydrogen the same thing here and the same thing here again I'm just writing the letter of the element the chemical symbol in that nucleus rather than drawing protons and neutrons because today I'd rather keep track of what atom that is rather than tracking protons and neutrons that really don't matter in this story so this hydrogen has an electron that it shares with carbon and this one has an electron that shares with carbon and now here's the story those electrons no longer belong to hydrogen or carbon they belong to both and they're orbiting so quickly that you couldn't freeze it and go nether with hydrogen neither with carbon they're truly orbiting both so these two electrons are orbiting around hydrogen and around carbon and same for all of them they are now shared so now carbon has a full outer electron shell and hydrogen has a full outer electron shell sharing of electrons each place that that is happening is called a covalent bond this oops didn't mean to do that sorry oh I'm sorry about that this is a covalent bond sharing one pair of electrons in that spot this is a covalent bond this is a covalent bond this is a covalent bond in this case we would call that a single covalent bond because those atoms are sharing one pair of electrons we're going to look at double and triple bonds today also triple is as many pairs as you can share a double bond is going to be sharing two pairs of electrons a triple is going to be sharing three pairs of electrons but you'll see that in a minute a simpler version that you'll more commonly see showing that sharing of a pair of electrons is every place there's a sharing a pair of electrons you just draw a straight line so a simpler version of this molecule this is a molecule because it's two or more atoms bonded together they're sharing electrons to become stable each one of these lines means sharing one pair of electrons so that is a covalent bond that is a simpler version of that molecule the chemical formula for this molecule would be CH4 subscript four that tells us it's one carbon and four hydrogens and they're bonded together this is a molecule in fact this molecule is called methane it is the simplest organic molecule organic molecule meaning carbon is in that molecule single covalent bond sharing electrons these bonds covalent bonds are very strong they are difficult to break and in fact they require energy input to form that energy gets stored in the covalent bonds so that energy input is now stored in the covalent bonds and when those bonds are broken specifically the covalent bonds is what we're talking about energy is released this is hugely important in biology because the way you get energy from your food molecules is breaking the covalent bonds of those food molecules the most important molecule that you eat is glucose glucose has a chemical formula of C6H12O6 that means that it's a molecule with six carbons 12 hydrogens and six oxygens all bonded together and they are covalently bonded together when you break the covalent bonds of that glucose molecule that energy is released and we repackage that as a molecule that's going to be the energy currency of the cell called ATP so in the cell we break the covalent bonds of glucose to form ATP and that ATP is the energy currency of the cell what that means is we need the energy from glucose to drive chemical reactions in the cell but we can't directly use glucose to do that we have to convert it to something that the cell can use and that's ATP kind of like when you travel and you're in a foreign country and you need to buy something you you need to exchange your US dollars for a different type of currency in order to be able to buy something in the cell we have to exchange glucose to a version of energy that the cell can accept and use and that is ATP that energy comes from the covalent bonds of glucose let's just quickly look at glucose in linear version you'll see this again in a future lecture when we talk about carbohydrates and you'll see it again when we talk about how we make ATP from glucose in a process called cellular respiration you'll learn about cellular respiration in detail in this class six carbons every one of those lines is a single covalent bond which means sharing one pair of electrons sorry I messed up there that flip right there is purposeful you're about to see our first double covalent bond okay a couple of things about this that are really important number one every time there's a line that is a covalent bond sharing electrons to complete the outer electron shells of those atoms this is a double covalent bond which is sharing two pairs of electrons to understand why two pairs of electrons are being shared in that spot oxygen is in column six it's atomic number eight it has six valence electrons it has six it needs eight to complete that outer shawl that means it needs two more electrons to complete this outer shell one way it can do that is to share two pairs of electrons with another atom now oxygen has a full outer shell carbon needs to share four pairs of electrons remember carbon has four in the outer shell that means carbon needs to share one two three four pairs of electrons to complete its outer shell looking at this Carbon on the end that carbon is sharing one two three four pairs of electrons so again this double bond counts is sharing two pairs of electrons let's look at the next carbon over one two three four this one one two three four the next one over one two three four sorry that was supposed to be right on top of that one next one over one two three four and then one more one two three four so every one of those carbons is sharing four pairs of electrons to complete its outer shell that is now a very stable molecule those bonds all contain energy and when those bonds are broken that energy is released and it can be converted to a currency that the cell can use to power chemical reactions in the cell combusting glucose to release that energy contained in the covalent bonds and convert it to ATP through a process called cellular cellular respiration is the most important chemical reaction that happens in your body in fact you make and use your body weight in ATP every day the second you stop making ATP you die you have to have a constant supply of ATP in the body it powers Every chemical reaction in the cell that requires energy and that's a lot of different things so covalent bonds are really important to understand in biology they're the strongest chemical bonds they require energy input to form when they're broken that energy is released and that is how some atoms can complete their outer electron gel and become stable is by sharing electrons to complete that outer shell so a double bond is sharing two pairs of electrons in a triple bond is three pairs of electrons so let's write all three kinds of covalent bonds here single covalent bond sharing one pair of electrons and a good example of that was methane double covalent bond is sharing two pairs of electrons and you saw that double bond in the glucose molecule but oxygen is also double bonded to oxygen to form O2 and lastly a triple covalent bond is sharing three pairs of electrons and that's the most two atoms can share remember that these bonds require energy input to form the double bond requires more energy than a single so a triple is going to require a lot of energy input and there's not enough energy available to form a quadruple Bond nitrogen is triple bonded to nitrogen that would be into that's what you need to know about the difference between single double and triple bonds you don't need to know any more than that there is another part of the story though that's important to realize and that is when two electrons I'm sorry yes when two electrons are shared so when two atoms are sharing one pair of electrons they're not always shared equally that becomes a little bit confusing but in the next video we're going to talk about hydrogen bonds and in hydrogen bonds there's covalent bonding that happens between atoms and that's going to set the stage for something else if those electrons are not shared equally so we're going to have equal sharing of electrons which is going to result in what's called a non -polar covalent bond and then the other option is unequal sharing of electrons which is going to result in what's called a polar covalent bond and I will explain that in the next video