now that we have learned about the basic structure of an atom and that it's the valance shell that determines the bonding behavior now we're going to study chemical bonds in module 2.2 atoms are combined by chemical bonds and a chemical bond is not a physical structure but rather it's an energy relationship or an attractive force between the two atoms now we have two definitions here a molecule is formed by chemical bonding between two or more atoms of the same element so an example of that is hydrogen h2 so this is a hydrogen atom this is a hydrogen atom and the bond that holds them together then creates a molecule another example of this is when oxygen bonds to another oxygen atom to create diatomic oxygen that's an example of a molecule so compare that to a compound a compound forms when two or more atoms from different elements combined so in this case hydrogen is bonded to carbon they are two different elements so technically this is a compound whereas if they're the same it's a molecule am i going to be spending a lot of time testing you on this concept no but this is just a good introduction and many of you will have to take chemistry later on now chemical bonds they form when the electrons do one of the following they either share electrons gain an electron or lose an electron there are three major types of chemical bonds which we're going to take a look at ionic bonds which are an attraction between a positively charged cation and a negatively charged anion and there's covalent bonds which are the strongest of the bonds and they occur when electrons are shared and the third type of bond is a hydrogen bond which are weak polar bonds based on partial electrical attractions so a hydrogen bond can only occur if the atoms have covalent bonds okay we're going to start off here with an ionic bond involves the gain or loss of an electron so the cation is the positively charged ion in order for it to be positively charged means that it has lost one or more electrons because electrons have the negative charge so if it loses an electron that means it's going to have more protons than electrons and it's going to take on a positive charge and it's going to be called a cat ion you can see it here in the middle with the t cross cat ion positively charged the other type is an anion this is our negatively charged ion an anion forms when the element gains one or more electrons and if you have more electrons than protons then you're going to take on a negative charge all right the attraction between opposite charges then bonds the ions to one another forming a compound called a salt so yes those compounds that we're going to study that are ionic in nature are salts this is a better way i think to do it so we've read through what ionic bonds are now let's look at how it actually happens so we're going to use sodium chloride as our example so here's sodium in its natural state so sodium has 11 electrons whereas chlorine has 17 electrons and the same number of protons so let's look at its valence shell sodium has one lonely electron in its valence shell so it has a choice remember in order for it to be stable and happy it needs to have a full valence shell so it can either find seven more electrons and add them to the shell or it can simply get rid of this one and then it will have a full valance shell chlorine has the opposite problem it has seven electrons in its valance shell so its choices are each either to find one or to get rid of seven obviously it's easier for sodium to get rid of an electron and it's easier for chlorine to gain an electron in order to find stability so here we are sodium's decided it's going to give up its electron and chlorine is going to accept that electron well once it gives up the electron now i have different numbers of charges i have 11 protons in the nucleus but now i only have 10 electrons so this ion is going to take on a positive charge of plus one that's our definition of a cation when the chloride takes on the additional electron now it has 18 electrons but only 17 protons so it has one extra negative charge making it an anion now i think everybody knows what happens when you put positive next to negative they attract each other and that's what creates your ionic bond so of the three types of ion to three types of bonds ionic is the second strongest covalent is the strongest let's talk next about covalent bonds so ionic bonds occur when one atom gains an electron and the other loses an electron we don't gain or lose electrons with covalent bonds instead we share electrons so down here we have an example of shearing electrons carbon is sharing electrons with four hydrogens now remember each hydrogen normally only has one electron so this one here is carbons this one here is carbons this one here is carbons this one here is carbons so in the outer shell carbon only has four electrons so it needs eight so it needs to gain four more so it shares one electron from each of the hydrogens collectively now carbon has eight electrons so technically it's happy when we share one pair of electrons as we do here between carbon and hydrogen that is called a single covalent bond if we take a look at oxygen oxygen has six electrons in its valance shell one two three four five six so how many more does it need to be happy two so when you have two oxygens sharing electrons they have to share two pair instead of one pair this sharing of two pair of electrons is denoted with a double line and that's considered a double covalent bond if i have to satisfy my octet rule by sharing three pair of electrons as nitrogen does then i depict that with three lines and that's called a triple covalent bond there are two types of covalent bonds nonpolar and polar the difference is whether they share their electrons equally or they don't share their electrons equally nonpolar covalent bonds result when electrons are shared equally so they're going to occur when the two atoms in a molecule have similar or identical electronegativities so this is a new word for you guys electronegativity is the desire of the nucleus for its electrons so what this is saying we've got three different examples down here nonpolar molecules occur in three situations when the atoms sharing the electrons are the same element so here we have two atoms of oxygen their or their desire for the electrons are identical because they're the same type of atom so one cannot have a greater desire and keep the electrons for a greater amount of time than the other because they are equals so since these electrons are going to move around both atoms an equal amount of time that makes it a nonpolar covalent bonds there will be no partial charges on this molecule a second example is due to the arrangement of the atoms in this case it's co2 carbon dioxide it is a linear molecule so realistically carbon has a lower desire or a lower electronegativity than what oxygen does in other words the nuclei of the oxygen atoms want the electrons more than the nuclei of the carbon but imagine these are sumo wrestlers we have one sumo wrestler on this side the other sumo wrestler on that side and they're fighting over the electrons that carbon has is either one of them going to be able to win no because they are equals because this is a linear molecule neither of them can take those electrons a greater amount of time than the other so this also creates a non-polar covalent bond our third example is whenever we have carbon and hydrogen the electronegativities of these two elements is so similar that they share their electrons equally all right so these are examples of nonpolar covalent bonds the type of molecule that i want you to remember as always exhibiting non-polar covalent bonds are lipids lipids typically share their electrons equally and lipids are often made up of lots of carbons and hydrogens now the other type of covalent bond is called a polar covalent bond and polar covalent bonds form polar molecules when atoms with different electronegativities interact resulting in unequal sharing of electrons so atoms with the higher electronegativity or the greater desire for the electrons become partially negative and this is the sign that you're going to see delta negative it's not a whole negative charge it's only a partial negative charge because this atom will keep the electrons around its nucleus greater share of the time than the other atom the atom with the lower electronegativity will take on a partially positive charge delta positive indicating a partial positive charge now water is the classic example of a compound with polar covalent bonds so oxygen has a strong electronegativity it has a great desire for electrons hydrogen not so strong not nearly as strong as oxygen so the electrons will spend more of their time around the oxygen than they do hydrogens since the electrons have a negative charge that means that the oxygen side is going to take on a delta negative or slightly negative charge and the hydrogen since they don't have the electrons as much as the oxygen does are going to take on a delta positive charge this is what a polar covalent bond is all about the unequal sharing of electrons creating partial negative and positive charges now the last type of bond that we're going to study is a hydrogen bond and a hydrogen bond can only be created when there are compounds exhibiting polar covalent bonds so a polar covalent bond is a precursor you must have that before you can have a hydrogen bond so here's your hydrogen bonds weak attractions between partially positive end of one molecule and the partially negative end of another molecule so we're going to once again look at water but instead of looking at the bonding within the water molecule because those are polar covalent bonds now we're looking at bonds which are created between whole molecules the delta negative end of the oxygen has a slight attraction to the delta positive end of the hydrogens and in this textbook we're always going to represent hydrogen bonds as dashed lines so whenever you see a dashed line that is a hydrogen bond hydrogen bonds are responsible for the following the attributes of water we're going to see in the next chapters that water is the universal solvent why is water so good at dissolving things because water has polar covalent bonds which allows it to create hydrogen bonds which are good at helping to dissolve various solutes into water hydrogen bonds are also responsible for protein folding proteins fold up to have a very specific shape so that they can do a very specific function what is it that allows proteins to fold into a specific shape hydrogen bonds and last of all hydrogen bonds are responsible for holding the two strands of dna together so here's an example of hydrogen bonds in action so here we have water and we have a glass slide a glass slide has no charges glass is made out of non-polar covalent bonds well when you put water onto a surface that has non-polar covalent bonds there is no interaction going on between the two so look what the water molecules do they are attracted to each other rather than being attracted to the air so they cling to each other and take on this circular shape whenever you take that drop of water and you put it onto the top of the glass slide does it spread out all evenly no it creates this hump because the water molecules are attracted to each other you have probably done this yourself when you fill a glass of water you can actually fill a glass of water above the rim just a little bit and why is that because the water molecules are clinging to each other via the hydrogen bonds here's another example dna is a double stranded helix so it's two completely different molecules that spiral around each other what holds these two strands of dna together hydrogen bonds between the nucleotide pairs and here's another example of hydrogen bonding creating surface tension so we've got two water bugs and actually sitting on top of the water rather than falling into it because the hydrogen bonds are holding them up review questions for module 2.2 describe an ionic bond an ionic bond is created when one atom loses an electron and becomes a cation and the other gains an electron and becomes an anion the two ions the cation and the anion one with a positive charge the other with a negative charge are attracted to each other and that's what creates an ionic bond a classic example of an ionically bonded compound is sodium chloride or table salt number two describe a covalent bond a covalent bond occurs when two atoms share pairs of electrons between each other number three what is the difference between a polar covalent bond and a non-polar covalent bond in a nonpolar covalent bond electrons are shared equally and there are no partial charges in a polar covalent bond one atom has a greater affinity or an electronegativity for the electrons than the other atom therefore the electrons will stay around one of the atoms a greater share of the time than the other creating partial negative and partial positive charges number four provide an example of a substance with nonpolar bonds examples would include oxygen carbon dioxide and lipids provide an example of a substance with polar covalent bonds that example would be water number five what is a hydrogen bond a hydrogen bond occurs between a partially positive and a partially negative location within a compound and why is hydrogen bonding important for the human body hydrogen bonding is responsible for the attributes of water making it a wonderful solvent it's responsible for protein folding so that proteins have a very specific shape and it's responsible for holding the two molecules of dna together in our nucleus