OK, we're going to do one here, this is called
ethene.
It's C2H4, and we want to write the dot structures
for ethene.
To do that, we always count our valence electrons
up first.
Let's take a look: Carbon is in group 4, sometimes
written 14, so it has 4 valence electrons.
If we come way over here to Hydrogen, it's
in group 1: it has 1 valence electron.
So 4 for the Carbon, but there's two of them
so we multiply it times 2.
Plus one for the Hydrogen, but there's four
of those so we multiply that times 4.
So 2 tiems 4 is 8, plus 4 is 12.
We have a total of 12 valence electrons here.
We're going to have to spread them around.
let's just write out--we always put the Hydrogens
(let's move down some here) on the outside.
So let's put our 4 Hydrogens on the outside,
and let's ignore this right here.
Let's put 2 electrons between each bond, so
they're chemically bonded, lower energy, more
stable, and then one in the middle.
So I've used 2, 4, 6, 8, 10, I've got 2 more,
so I'll put those out here.
But my problems is that I've got 2, 4, 6,
8 for that one, but this Carbon only has 6.
So I can't, with those 12 electrons, satisfy
the octet.
So what I'm going to do is take these and
stick them right here in what's called a double
bond.
Now if I cover up this side, I've got 2, 4,
6, 8.
If I cover up this side, I've got 2, 4, 6,
8 and I've only used 2, 4, 6, 8, 10, 12.
So I used my 12 electrons to form a double
bond here that gives Carbon an octet; and
then Hydrogen, they each have 2, so their
outer shells are full.
Sometimes you'll see that written like this:
the double bond showing you the two pairs.
I suppose you could write your H's; often
they'll show the single bond like that with
the H's.
That's kind of an abbreviated format for the
dots.
So there you have it: ethene.
