Rob Lederer: So, Hydrogen, hydrogen has one electron. We know that it was one zero zero plus one half for a set of quantum numbers. Now all we have to do to describe where hydrogen's electron is is draw something called an electron configuration. That's what we call them. One, because that's the end number. Now one zero was the L value for the quantum number. So, that's abbreviated with the letter s we just saw. So, its one s. Now, how many electrons does it have in the one S orbital, this hydrogen its got one electron there, that plus one half, so there we go. Hydrogen is 1s1 and that's called the electron configuration. Now look at helium, helium's electrons, we know that they were two in the one S orbital. Its one zero zero plus one half and one zero zero negative one half. So, you go 1s2 and how was the electron configuration for helium. Oxygen, oxygen was 1s2 filled that s orbital and now we have to go to the next one. Now remember, N equals two L still equals zero and zero is in s. So, 2s. How many electrons fills the zero? Well, only a maximum of two. So, it was 1s2, 2s2.Now we went from two zero to two one in the set of quantum numbers to get the electron quantum number sets for electrons numbers three through eight. So, what do we going to do? That's actually electrons five through eight. So here we are going to do now to describe the energy level two and the orbital, we go from s to p and so its p2s 1s2, 2s2 2p, how many electrons left? There are four left and all of them went into the two one set of quantum numbers. And so its 2p4. You want to make this a lot easier? Watch how it works on the periodic table and becomes a snap; this is so beautiful. It really is. You know, we might not think I get out one, okay. But this really, this is just exciting. Hydrogen is 1s1. Helium has its two electrons, 1s2. Then lithium is 1s2 2s1. 1s2 2s2, Is2 2s2, but the outermost electron now must be 2p1. Outermost electron 2p2, 2p3, 2p4, 2p5, 2p6, then where is the outermost electron for sodium? Its at N =3 in that set of quantum numbers. So, it's a 3s1, outermost electron 3s2. Then 3p1, 2, 3, 4, 5, 6. You get the idea? Look, on periodic table, here is how we block them. Hydrogen and helium are in the Is. Then this is a 2s block. Then this is 2p. This is 3s. This is 3p. So, all of the p block of the periodic table is here. This is sectioned of as the p block. These are the s orbital blocks here where the outermost electron is in s orbital here, p orbital in here. But, what about this section in here? Scandium, watch me give it its electron configuration. Is2, 2s2, 2p6, 3s2, 3p6, 4s2 but you know at N equals three, we didn't finish that because, that N equals three, L can be zero one and two. That's what the L is. And that's stands for s, p and d. There are d orbitals at N equals three. Where is the d block? Right here and the d block starts that three here and then goes 4d and 5d. So, this is the 1s, 2s2, 3s, 4s, 5s, 6s, 7s that s block. Over here, we have got 2p, 3p, 4p, 5p, 6p but in the ds, the d start at three. Why did that d start at three because, at N equals two, you can only have L equal to zero and one? That's s and p. There are no ds at N equals two. The d start at N equals three and its this section here and if you actually, now stay with me, okay if you actually go N equals three, L equals and then we go all the way to two. Then that means that the NL can equal negative two to positive two. Negative two, negative one zero, one and two. That's five set orbitals that can hold a total of times two, 10 electrons. Oh! My Goodness, look at this. 1, 2, 3, 4 , 6 , 7, 8, 9, 10 elements in the d block. How many electrons can be in the p orbitals hold? Six. 1, 2 , 3 , 4 , 5 , 6. It's beautiful. 50 years before anybody ever came up with orbitals and things like that, Mendeleev made his periodic table like this because, he just grouped elements together with similar properties going down and increasing atomic mass going across.