welcome to sections 13.2 and 13.3 all right gentle people in the last lecture what we covered was ionic bonds and we revisited this topic and just to remind you ionic bonds are made out of two ions coming together a cation and an anion now this can involve polyatomic ions coming together with an ion but a very good indicator for an ionic bond is a metal bound to a non-metal now if you will recall we talked about another type of molecule and that molecule was put together using covalent bond you identified these when you saw two non-metals coming together now a covalent bond means that two elements are sharing electrons together so here are examples of things that are forming covalent bonds hydrogen with hydrogen chlorine with chlorine and oxygen with oxygen so let's go ahead and explore how this interaction works so to help us understand this what i'm going to do is i'm going to take two hydrogen atoms let's call this hydrogen atom a and hydrogen atom b now remember what a hydrogen atom is made out of there's a proton in the center and then the electron is around that proton so let's start out with these two hydrogen atoms infinitely far apart if that's the case let's go ahead and say that the energy of our system is zero now what i'm going to do is i'm going to bring these two atoms closer and closer together now what's going to happen is that the electron from hydrogen a is going to see the proton in hydrogen b and the opposite is true the electron from hydrogen b is going to see the proton in hydrogen a now remember an electron seeing a proton is a favorable interaction a positive and a negative want to be close together and so what happens is i get a lowering of energy of my system because now now each electron can see two protons and each proton can see two electrons and so as i bring them closer and closer together these new interactions become stronger and stronger and i lower the energy of my system now remember nature always wants to lower the energy of the system so this interaction is a favorable interaction and it's better for these two atoms to be together than they are apart however there's a limit to how close i can bring these two things together what you guys will see as i bring them closer and closer together you can start to see that i start to raise the energy of our system and this is because that if i bring these atoms too close to each other the proton from a will start to see the proton from b and i don't want to have two positive charges seeing each other and so there's an optimal distance that i can bring these two atoms together and that is when i maximize the electron from one atom seeing the proton of the other atom and minimizing the two protons seeing each other this turns out to be the bond length or the distance between the two nucleuses so what you should take away from this is that if we start to share electrons i can go ahead and lower the energy of my system and that's going to be the rise of the covalent bond where i have a sharing of electrons to lower the energy of our system and this is a common occurrence when i bring two non-metals together now if we look at these molecules that i've underlined you'll notice that i'm using the same atom coming together these are homonuclear molecules meaning that there's only one atom forming these molecules however we can go ahead and bring different atoms together but there are consequences when we do this one of the characteristics that arise when you have two different atoms together is the possibility that the bond becomes a polar bond so what you guys will see is if i form this molecule hcl these two different atoms come together and share electron if i were to go ahead and pump hcl gas in between two charged plates so one plate has a negative charge and one plate has a positive charge what i would see is the molecules would align along those plates what i would see is that the hydrogen would be pointed towards the negative plate and the chlorine part of the molecule well that would be pointed towards the positive plate what this is showing is that this hcl molecule is acting like a bar magnet and we've discussed this concept before in a previous lecture what's happening here is that the electrons are not being shared equally in this case chlorine is being a greedy atom and so what it's doing is it's pulling all the electrons towards it even the one that is shared by hydrogen now it's not completely ripping the electron away from hydrogen it's just trying to pull it closer to itself when hydrogen is trying to share its electron so what i can draw is what's called a dipole arrow in chemistry when we draw a dipole arrow we draw an arrow towards the more greedier atom in addition on the tail of this arrow we're going to make it a little cross what this signifies is the positive and negative end because the chlorine is greedier and pulling the electrons towards it it adopts a slightly negative charge so we're going to use the small delta symbol and a minus to denote a partial negative charge now this isn't a full negative charge like if the electron were removed from the hydrogen and placed on the chlorine it is only a partial charge because the chlorine is just drawing the electrons closer to it on the other end of the molecule we're going to have a partial positive charge that's because the hydrogen doesn't get to experience that electron on it in the full degree if i have a net dipole we consider the molecule polar and it behaves like a little bar magnet you'll note the dipole arrow because of its cross the cross can look like a plus sign telling you which is the positive end of that bond the arrow tells you the negative end of that bond now one of the questions that you might ask is how do we know which atoms are greedier than the other atoms and this has to do with a characteristic called electronegativity electronegativity is the ability for an atom to draw electrons towards it when it is part of a compound meaning it is in a bond now i want you guys to be careful this is different from electron affinity electron affinity means i'm putting an electron onto an atom electronegativity has to do with how the atom acts when it is bonded to something else we can get the relative electronegativity of atoms by doing experiments and what you guys see here are the values of those electronegativity now there is a trend on our periodic table what you guys will see is that fluorine is the most electronegative atom as you go down from fluorine you become less electronegative and as you go from right to left away from fluorine you become less electronegative now we can calculate the difference in electronegativity so that's going to be delta electronegativity go ahead and look up the values of each one of your atoms so in this case we have chlorine at 3 and hydrogen at 2.1 so the difference in electronegativity is 0.9 in general when you put compounds together if the electronegativity is is really great i.e greater than 1.8 this is probably going to form an ionic bond if it's between 0.4 and 1.8 these are considered polar covalent and if it's less than 0.4 we would say it's mostly covalent meaning it has a very low polarity if it is zero then this means it's a non-polar molecule so if we look at this calculation hcl is considered a portal or covalent bond all right jenna people why don't you go ahead and take a look at this quiz question and give me an answer you guys can use the electronegativity values on the previous slide to help you make this assessment all right looking up the values we have chlorine at 3.0 minus 0.9 for nacl we get 2.1 you guys can say that nacl is an ionic compound and this electronegativity difference corresponds to what we would pre believe assessed nacl as lih 2.1 minus 1.0 1.1 is our resultant electronegativity difference for hf we have 4.0 minus 2.1 this gets us to 1.9 and finally rubidium oxide 3.5 minus 0.8 which gives us a value of 2.7 so because the last one has the highest electronegativity difference it is considered the most ionic compound well i hope that made sense chem 1a and remember to stay safe