 Hello, this is Professor Stephen Esherman here. I'm here to talk to you a little bit about shells, the periodic table, and an introduction to atomic orbitals. So I'll start off here. If you can just remind you of the connection between shells and the periodic table. Here's the periodic table. Here's the first row, which has one shell. Second row has two shells and so on. And what next topic I just want to tell you about then is that shells are actually made up of things called orbitals. So inside each shell there are subdivisions of the electron cloud that are called orbitals. And an orbital tells you where you're likely to find a particular electron. They come in lots of different sizes, orbitals do, including one that looks like a sphere like this one here on the right, which is called an s orbital. And this is just a schematic way of showing that there is an electron that's in that orbital. In this case there's only one electron in that orbital and since the maximum is two, as we'll see in a moment, we would call this a half filled s orbital. Now if there's two electrons in an orbital then we draw two arrows pointed in opposite directions and so here's how we would describe a filled s orbital. And there are lots of more orbitals on every atom. Most don't have any electrons and we would just call them empty orbitals. So there is this idea that a maximum of two electrons can fit into any given orbital is the first part of a principle that's called the poly exclusion principle. So let me flesh that out a little bit for you. The idea is that every electron is actually said to be spinning and turns out electrons can only spin in two directions like you know counterclockwise and clockwise if you like. And we'll just call one of those spin up and the other one spin down. And here's the deal is that electrons if there are going to be two electrons in the same orbital they have to be what we call spin paired which means that they have opposite spins. So that's why I put two opposing arrows here in this s orbital. It's a filled orbital with spin paired electrons. Just as a counter example this doesn't happen because they have the same spin. That's not allowed by the poly exclusion principle neither does this, same problem and here's a bigger problem that we've tried to cram three electrons into that orbital and that's not allowed by the poly exclusion principle. So I'm just going to take you for a little tour over some of the elements in the various rows. So back to that diagram here's a here's the first row or period of the periodic table. It turns out the first shell only has one orbital it is an s orbital it has a name it's called the one s orbital and therefore the first shell has a maximum according to the poly exclusion principle of two electrons provided there's spin paired. Now since that is the only shell and it is the outermost shell because it's the only shell then those electrons that are in that orbital that one s orbital are called valence electrons they get a special name. Second row okay so here I've drawn two shells and I've drawn here there's the inner shell that's the first shell it still only has one s orbital it has two electrons but now because that's not the outermost shell they get called core electrons and here we are in the outermost shell it's just two of all of them uh in the second row um turns out that shell has four orbitals which means it can hold a maximum of eight electrons again to the do the poly exclusion principle. Those are now going to be called the valence electrons because they are in the outermost shell and the electron the orbitals that are there there's one two s orbital that's a sphere and then there are three 2p orbitals that are shaped like these double dumbbells. Now um just a kind of a point I don't know if you notice about this but when we are talking about going thinking about a one s orbital and then in the first shell and then now we've gone to the two s orbital in the second shell uh they look the same they have the same shape but um generally the orbitals get bigger uh as you get to higher shells and in fact that's why the shells are bigger that's because their orbitals uh are bigger. All right um I also just need to have a little caveat here that depicting shells as concentric circles is a little bit misleading because again I'm going to go to neon here which has two electrons in the first shell eight electrons in the second shell for um a total of 10 electrons so there's two here two here and then a total of six here those are the 3p orbitals. The fact is they're all piled on top of each other so it's not really concentric rings it's really a big old dense ball of of electrons but it's still convenient to write them as those concentric circles. All right let's go to the third shell uh third row I mean of the periodic table uh that has electrons in shells one two and three so all the electrons in the first and second shells are now going to be called core electrons the electrons in number the shell three are called valence electrons and uh uh they are once again it's a now a three s orbital that three refers to the shell just like the previous ones did and now we have 3p orbitals and uh and so for a total of eight more electrons and you can count two for so on um and uh so here's another point it's really to kind of keep your your yourself organized on this I recommend that you copy this general order energy ordering diagram into your notes and memorize it and it's energy on this axis here's that first shell which has a 1s orbital here's the second second shell which has those four here's the third shore uh shell that has uh also four orbitals we're going to ignore those guys there for the moment and let's see oh well and there's an idea that uh we've used so far but have it named is called alf bow and alf bow is the mental process of assigning electrons to the lowest available orbitals while honoring the poly exclusion principle so the result of that is this kind of a shorthand called an electron configuration and here the electron configuration we would just call it one s with a superscript two that means there are two electrons in that one s uh orbital uh which is helium so a math pseudo mathematical way of saying that would be the electron configuration of helium is one s superscript two now alf bow usually results in the lowest possible electron uh atomic electron configuration something like this called the ground state because it is the lowest energy uh state of the atom um that's not to say that other configurations are impossible they are certainly possible but they all get called excited states and i'm mentioning this because um back in the 1800s uh Heinrich Geisler um put together this sort of uh this instrument uh which consists of a tube it had some various gases in it he was able to apply voltage to the two um two electrodes on either side and then discovered that when he did that it would glow okay and you're kind of familiar with this because neon signs uh this is basically the neon gas that's in in uh in a geisler gas discharge tube now not all of the light that comes off in those situations is visible to humans uh some of them uh some of the photon some of the light that that that comes off is uh is is in the infrared range which we can't see and some of it might be in the ultraviolet or the x-ray range i'm going to talk a little bit about how that happens so here's back to helium and its ground state and uh here's just that same diagram and so here's helium just sitting there as ground state electron configuration but now we're going to zap it with some with some voltage and what happened was you can see that that electron down here got bumped up way upstairs to this 3p orbital so now it's new electron configuration looks like this 1s1 3p1 okay now it's an excited state so it doesn't stay there uh it will that electron was there and now it might drop down to a slightly lower uh orbital which in this case is a 3s orbital so that's its new configuration and uh and when it does so it emits a little flash of light which we're calling a photon now the color of that emitted light i mentioned isn't always in the visible range it totally depends on the energy gap from where the electron was to where it's going so if the energy gap is between 1.8 and 3.6 electron volts which is the unit of energy the emitted light is visible to to humans and there's lots of gaps possible so there's lots of colors of light that you might get out of a out of a geisler discharge too now we can out bow all the way up to neon we could say um all right so helium had two electrons here but neons got eight more electrons to its element number 10 so it has 10 electrons so we could pop two here and then a total of six there and so if you want to think about what the ground state electron configuration could be you could pause write it down and that is what i get the same electron configuration of neon the same as helium followed by two electrons in the 2s orbital and six electrons there now we could even out bow all the way up to argon so we could say okay argon has 10 more electrons because it's element um sorry eight more electrons because it's element 18 and if you want to think about what that might be all right i think it's going to look like this it's going to look like the same as neon build uh core there followed by a couple of electrons in that 3s orbital and then six more there let's see the next orbital up in the diagram is after argon there's a 4s orbital that's in the fourth shell you can hold two more electrons so that would be elements 19 and 20 and by the way we're here there was argon and then we're at potassium and calcium 19 electrons and 20 electrons you could probably figure this one out but there's potassium it's looks just like argon with one electron in that 4s orbital and calcium got two there electron paired let's see um the 3d is next because you can kind of see here but i'm still going to reference it to argon in other words uh there the next set of orbitals is a set of five d orbitals this is what they this is what they look like and so now we're over here on what are called the transition metals let's see uh scandium i would say well it looks just like argon plus two electrons in that 4s plus one electron in that in that 3d because uh because scandium is has 21 electrons