All right.
Let's look at that covalent bond.
And what we have to do is, of course,
start with our electron configuration.
So what I like to do is take a look
at a molecule that is held together by covalent bonds.
So let's look at methane-- that's CH4, methane.
And sometimes you may have seen it sketched,
or you will see it sketched, like this, in one plane,
in two dimensions.
That's not actually what the molecule looks like,
but it's a decent way of depicting it
for these purposes.
So you've got carbon.
These solid dashes connect to these hydrogens,
and that's one molecule of methane, CH4.
But if we want carbon to bond to four hydrogens,
how are we going to do it?
So what we do is, let's start with the atomic number.
And so for carbon, it's six.
And so we know we've got 1s 2 2s 2 2p 2.
So there's four valence electrons.
And we need, or we would like to have, eight in total
to get that stable octet.
So how are we going to get to the stable octet?
Well, if we draw carbon and, say, for a simple model
we're just going to draw the electrons that carbon
brings to the table.
There they are.
And "covalent" implies some sharing.
So there's sharing of electrons.
Valence electrons are the outermost electrons.
So covalent means they're the outermost electrons
for two nuclei.
Shared.
And so let's introduce hydrogen. Here's
hydrogen. It's got its one electron.
Hydrogen-- one proton, one electron.
And if hydrogen gets together with this carbon,
and they share, well then, hydrogen
appears to have its stable electron configuration.
Which actually, hydrogen wants-- if you now
allow me to anthropomorphize hydrogen--
to look like-- its hero in the world is helium.
It wants to be like helium, and helium is just 1s 2.
That's the exception to the octet.
It's duet stability, I guess you could call it, for helium.
Hydrogen wanting to look like that.
But carbon, though-- I'll draw a little blue circle around here,
just to show.
There's those eight valence electrons.
By sharing with four hydrogens.
So the covalent bond involves sharing of electrons,
and it's also between specific atoms.
It's a term that sometimes we would use here,
and that term is "directional."
That is, if we go to this model here,
there's a direction to it.
This hydrogen and this carbon specifically
are involved in the bond.
And if I drew another methane, say--
let me just redraw them down here--
it's between specific atoms.
So we've got carbon here, with those hydrogen
in one molecule of methane.
If I drew another molecule of methane here,
there would be no covalent bond between this hydrogen
and this one.
That will be a secondary bond.
We'll get to that later.
But it's specific.
And not only that, that'll be an interesting contrast
to the ionic bond, which we'll cover later.
Also, because it's shared, you can say,
is a covalent material going to be conductive?
Well, the electrons are bound to these nuclei.
So here we go.
This carbon has got a hold on those eight electrons.
They're bound tightly to the nuclei.
And so there's no electrons available
to conduct electricity, or even heat.
And so covalent materials are typically insulators,
non-conductive.
And the class of materials that's
characterised most clearly by covalent bonding is a polymer.
OK.
Now there are covalent ceramics, as well,
but typically, the best example, if you will,
of covalent material is a polymer.