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# Electron configurations for the third and fourth periods

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- [Voiceover] We ended the previous video with the electron configuration for neon. So, 1s2 2s2 2p6. I showed you how you could
look at the periodic table and kind of run through these
electron configurations. For example, this would be 1s1, 1s2 and then 2s2 would be here and then we had six. So 2p6 brings you all
the way over to neon. And so for an electron configuration for the elements in the third period, so this would be the first period, second period, the third period. So let's do sodium. Sodium has 11 electrons
so one more than neon but the second shell is full. The second shell is completely full. So for sodium's 11th electron we need to go into the third shell, into the third energy level. And so, n is equal to
three for the third shell. Possible values of l include zero, one and two. In the third shell we're talking about an s orbital, right, one of them. P orbitals, right, 3p orbitals and when l is equal to two we're talking about a d orbital. If you do your magnetic quantum number you get five values for that. So we're talking about five d orbitals in the third shell, in
the third energy level. And so if we plot those orbitals, all right, so let's go and plot those in terms of increasing energy. This side we'll put increasing
energy going this way and the 3s orbital is here. So that's what we're talking about. We're talking about the third shell. We're talking about the s
orbital in the third shell and there's one of them. All right next, we have 3p orbitals, in the third shell. Let's go ahead and draw
in those p orbitals, so there's three of them. So one, two and three. Those are our 3p orbitals. And then finally we have some d orbitals, we have five of them in the third shell. So five d orbitals. Let me go ahead and draw those in. Those are our higher energy. So one, two, three, four and five. Here's the 3d orbitals like that. All right, let's do sodiums. Let's go back to sodium down here. 11 electrons. All right, so we need... Sodium has 11 electrons and so the first 10, we could put the first 10
electrons in just like neon. 1s2 2s2 2p6. And remember your superscripts tell you the total number of electrons, that's two, four and then 10. So that takes care of 10 electrons. We have one more, one more to account for and so, the 11th electron for sodium is going to go into the third shell and the lowest energy here
will be the 3s orbital. So we go ahead and put the
11th electron for sodium into the 3s orbital so we can complete the electron
configuration for sodium and we have to add on 3s1. Because we have one electron and an s orbital in the third shell. The complete electron
configuration for sodium becomes 1s2 2s2 2p6 and 3s1. Notice that all of these, 1s2 2s2 2p6, this is the same electron
configuration as neon and so we could represent all of that, we could represent all
of these right here. We put neon in brackets and then we could write 3s1. And this is another way to write an electron configuration for sodium. So we call this noble gas notation because we're using the noble
gas that precedes sodium. So if you just work backwards
in the periodic table, you could go backwards from sodium, the first noble gas that
you hit here is neon. And so we do neon's electron configuration is the same as neon's and then we have to add on 3s1. Neon 3s1 is our noble
gas notation for sodium. Let's just do another
element in the third period. Let's just go all the way
over here to aluminum. So 13 electrons for aluminum. We can use noble gas
notation to save us some time so we're saying the electron configuration is the same as neon. And that puts us right here. Let me use a different color. All right, so that puts us right here. And then we have sodium, sodium would be 3s1,
magnesium would be 3s2 and then let's go ahead and put those in. Magnesium would be 3s2,
so we fill that in here and then we need one more
electron for aluminum. We need one more electron and of course that electron goes into one of these p orbitals here. And so let's go ahead and write
the electron configuration. Just look at what we have on our orbital notation here. So we have two electrons
in the 3s orbital so 3s2. And one electron in one of the 3p orbitals so we write 3p1. So, brackets neon 3s2 3p1 is the electron
configuration for aluminum. Let's go ahead and do... Let's jump all the way over to argon here. So let's go all the way over to argon. Let's write the electron
configuration for argon using noble gas notation. The noble gas before argon is neon. So we put neon in brackets and then once again we think, we could think this is 3s1, we could think this is 3s2. So we have 3s2. We go over here this is 3p1, 3p2, 3p3, 3p4, 3p5 and 3p6. We can go ahead and write 3s2 3p6 here for argon. And if we want to put in those electrons and how they fill, we could put in those
electrons following Hund's Rule and we talked about in the last video. And then we put in all of those electrons so we can see that we've now filled the 3p orbitals like that. So that's argon. Next up is potassium. So now we're at the fourth period on the periodic table. So if we go one more element here we hit the fourth period
of the periodic table and we get to potassium. You might think, all right, so one more electron than argon, so you might think potassium's electron would go into into a d orbital, because that makes sense. We have this d orbital here but that's not what happens. So let's go ahead and take that electron out of that d orbital. We are now, we're in the fourth period and we can actually open
up a new shell here. We can go to n is equal to four. We can go to n is equal to four, let me go ahead and write this down. And in the fourth shell, I won't draw, I won't write
in all the values for l but we definitely have an s orbital. So there's an s orbital in
the fourth energy level, one of them and it turns out the energy for that 4s orbital is a tiny bit lower than the 3d orbitals. Let me draw that in with
a different color here. The 4s orbital turns out to be a little bit lower in energy. Let me use a different blue so we don't confuse it here. This is representing our 4s orbital. For potassium, potassium this actually... The 4s orbital is a
little bit lower in energy so the next electron
that we add for potassium is going to go into this 4s orbital. We can go ahead and write
the electron configuration for potassium. If we're using noble gas notation, we go backwards. So what's the noble gas
that precedes potassium? Just go backwards in the periodic table and that is of course argon, that's argon right here. We could say that the electron
configuration for potassium is the same as argon's and then we have one more electron to worry about here. That extra electron, that
19th electron for potassium is gonna go into this 4s orbital here. We write the same electron
configuration as argon and we write 4s and one
electron in that 4s orbital so we write 4s1. For potassium, one more electron. We can go ahead and write... Sorry, for calcium I should say. So for calcium, one more
electron to worry about so the noble gas that precedes calcium is once again argon. So we say it's the same
electron configuration as argon, one more electron and we
know that there's space in our 4s orbital here. We can add another electrons pair spins up and so we can say 4s2 here for the electron
configuration for calcium. Once again, we notice this
pattern on the periodic table. This right here, let me use a blue here. We could say this is 4s1,
we could say this is 4s2. All of these over here on the left we have called... Thought about them as
being s orbitals anyway. So this is the s block
on the periodic table. All right, that takes care of calcium and the next ones that you're going to hit are all of these elements in here. So all of these elements. We know that in the third energy level there are d orbitals. So let's go back up to here. Let me ho ahead and
mark this so we can see. We know there are five d orbitals in the third energy level. Each d orbital can hold a
maximum of two electrons so five times two is 10. How many spots do we have here? One, two, three, four, five, six, seven, eight, nine, 10. We have these 10 spots here. I'm actually gonna do
a whole separate video on the d orbitals so I'm not gonna worry about them in great detail right now but we have 10 spots. This is the third shell and we have 10 spots for electrons. So 3d10 would fill these d orbitals. 3d10 fills these d orbitals here. And this is why the periodic table is shown the way it is here. It just helps you to think about writing your electron configurations. For example, let's just
go ahead to gallium here. Let's write electron
configuration for gallium. So if we're using noble gas notation, the noble gas that precedes gallium if we work backwards would be argon. So we can go ahead and write that in here. That takes us to this area. That would be 4s1 4s2. So we write 4s2 right here and then we have all of our d's. We can just go ahead and
put the d's in there, 3d10. So we put in 3d10 here and this takes us to... We're in the fourth shell here and it's equal to four so l is equal to one
is another possibility. And so those are the p orbitals. Once again, we have 3p
orbitals in the fourth shell. Of course right here, this is
where we hit the p orbitals. We start to fill the p orbitals. We can go ahead and
say this is a p orbital in the fourth energy level. A p orbital in the fourth shell and we have one electron so we can write 4p1. And so this is one way to represent the electron configuration for gallium using noble gas notation. So argon 4s2 3d10 4p1. Sometimes you might see the 3d10 and the 4s switched in
terms of their orders. You might see 3d10, 4s2, 4p1. It doesn't really matter how you do it. You'll see both ways done. All right, finally let's do krypton here. Let's do krypton. Our last electron configuration here. The preceding noble gas would be argon and let me use a color
that we can see here. Argon will take us to here and then we have 4s2. So we fill 4s2 and then we have all of our d orbitals here. So 3d10. So 3d10 takes us to here. Now we're in the fourth
shell in the p orbitals so one, two, three, four, five and then six. So 4s2 3d10 4p6 would be one way to represent the electron configuration for krypton. And once again, you could switch the 3d10 and the 4s2 if you wanted to. You could say for krypton the electron configuration,
the noble gas notation would be argon 3d10 4s2 4p6. Just do whichever way your professor wants you to do it. That's covering the third
and the fourth period but we've ignored the d orbitals. Again, that will be a whole separate video because it gets a little bit complicated when you get into d orbitals.