All right.
So let's take a look at some electron configuration
examples.
And why don't we start with carbon?
So carbon.
Look it up on the periodic table,
and you'll find that it's got a atomic number or proton
number of 6.
So there's 6 protons.
We need 6 electrons to make a neutral atom of carbon.
So we're going to go ahead, and we're
going to start filling from lowest energy to higher energy.
And so there's this little memory aid
that you can use to remind you of the electron filling
orders or the relative energy levels of these orbitals.
For s-- we've got s, p, d, f as these orbitals
and increasing one, two, three, four for principal quantum
number.
And we're going out-- I mean, we don't really
need it so much for carbon.
But if I go through it-- just to be robust.
And so carbon.
We need 6.
So we're going to fill 1s first.
And 1s, since it's an s orbital, it can have 2 electrons.
Spin up.
Spin down.
And we're going to fill 2.
Again, same story, it's an s.
2p.
And so we've got 2 plus 2 is 4.
We need 2 more.
So 2p 2.
So that's the electron configuration for carbon.
Let's take a look at another example.
And this time, actually, let's look at neon.
OK.
That's an inert gas, a noble gas.
And it's got an atomic number of 10.
So we're going to, again, go through this same process
we had over here, and we're going
to start to fill electrons.
We've got 1s 2 2s 2 2p 6.
We can actually fit 6 electrons in p.
Right?
That's what this box diagram reminds us of.
OK.
I'll spin up and spin down in each.
That gives us 6 electrons.
So this is what we've got at this point,
and that is the electron configuration for neon.
Now, neon's very unreactive, and it's interesting
pay attention to the fact that this
has got a full p and a full-- in fact, a full 2-- correction, 2s
as well.
Right?
Because that little guy is full as well.
Why don't we take a look at-- let's go a little heavier,
and let's look at argon.
All right.
Argon.
There's argon.
And it's got an atomic number or proton number of 18.
Interesting.
So I can write this out.
1s 2 2s 2.
Again, we're just going through increasing energy levels here.
2p 6.
So that's 10 electrons.
I need 8 more.
And if we filled 2p, we're going to fill 3s next and then 3p.
So we're going to fill a 3s.
It's going to have 2 electrons in it.
And then 3p, and that's going to have 6.
And that's 18.
Now, a couple things that are worth mentioning-- or one
thing that's worth mentioning at this point
is it was rather tedious perhaps to listen to me go through
and write out those initial electrons.
We'll call them "core electrons" a little bit later,
and those are the ones closest to the nucleus.
So in fact, if we want, I could to say, well,
that looks-- that's neon.
So a shorthand notation that's often used
is we say neon, put it in square brackets like that,
and then just add the outermost electrons.
OK.
And so the outermost electrons, of course,
are called the "valence electrons."
So we can just write the valence electrons
and then wrap the core electrons up into that inert gas.
Again, though, you'll notice a pattern here.
Right?
We've got 2s 2.
2p 6 for neon.
For argon, 3s 2, 3p 6.
And in fact, there is this arrangement with 2 plus 6.
Equal to 8 is a very stable electron configuration.
It's known as the "stable octet."
OK.
Or "octet stability."
And it-- what we've observed is that, if you
have for a given principal quantum number-- say,
n for this particular shell-- if you've got 2 electrons in the s
and 6 in the p, that electron configuration is
particularly low energy.
And atoms will try to achieve this low energy electron
configuration, and that's the basis for bonding.
We're going to see that atoms start
to participate in bonds in order to achieve this electron
configuration.
So in fact, if we back to carbon, you'll see-- well,
how far away from that stable octet is carbon?
Well, it's got-- how many valence electrons does it have?
It's got 2 plus 2 is 4.
So you may-- and I'm jumping a bit ahead here--
but if you've ever seen carbon written like this,
say, with hydrogens all around it and a methane molecule,
well, that is showing you that you're
making four bonds to form that stable octet
and get 8 electrons around the carbon.
So that's the stable octet.
I hope that was useful.