Covalent Bonding
Consider the Chlorine
A single, neutral chlorine atom has 7 valence electrons: [Ne]3s 23p5
As a Lewis Structure (aka, just an element symbol with a dot for
each valence electron), chlorine looks like this:
Cl
It is a sad chlorine. It wants to have
the same electron configuration as
a noble gas, which it can do by
either losing all 7 electrons
(unlikely), or gaining one (much
more likely).
Octet Rule: everything wants eight
valence electrons
One Solution
Like my mother (never) said: if you want something, just steal it.
Chlorine can find an element that's looking to give away some
electrons. Sodium, maybe.
Cl
Na
Because sodium has just one
valence electron-- [Ne]3s1—the
easiest way for it to get to a noble
gas configuration is to lose one.
So...
One Solution
YOINK!
Cl-
Na+
Now everybody is happy. The
chlorine is [Ne]3s23p6 (aka [Ar]), and
the sodium has the electron
configuration [Ne].
This gets us an ionic compound (in
this case, NaCl)
But What If There's Nothing Else?
If there's literally no other elements around, our sad chlorine is in a
bit of trouble. It can become 'happy' by taking an electron from
another chlorine, but that just makes that chlorine even worse.
Cl-
Cl+
Of course, that chlorine could take
two from another chlorine, but that
makes that chlorine short three.
THAT WAY LIES MADNESS!
6 electrons!?!
But What If There's Nothing Else?
Like my mother (actually said): share with your brother or I'll just
take it away from both of you.
Cl Cl
By sharing one electron each, both
chlorines still only have seven
valence electrons that belong to
them, but they have eight valence
around them—same as the noble
gasses.
They've made a covalent bond!
Bond. Covalent Bond
Covalent bond: two valence electrons shared between atoms.
+
-
+
Because the nucleus is positive, if
you put some negatives between two
nucleii, they will both be attracted to
the negative charges, and therefore
held together.
Since there are bonds, this is a
molecule.
What About Oxygen?
With only six valence electrons, oxygen can't get to eight just by
sharing an electron each:
We're still sad. :(
O O
But if you share two electrons each, both oxygens will have eight
valence electrons around them—just like the noble gasses. You
make two bonds (aka a 'double bond').
O
O
You can see how this could get kind of
messy and confusing as molecules get
bigger, so there's another way to draw Lewis
Structures
Line It Up
Drawing bonds as lines is a little more simple visually, and also just
moves the atoms farther apart.
O
O
Each line represents two electrons—one from each atom—being
shared to make a bond.
Each pair of unshared electrons is referred to as a lone pair.
Mix and Match
There's no reason we need to limit ourselves to two atoms, or to two
of the same type of atom:
O
Cl
F
Like chlorine, fluorine has seven valence electrons, so it needs
to gain one, and therefore make one bond. The oxygen, with six,
wants to make two bonds.
Cl
O
F
(Don't breathe near this molecule.
In fact, just don't be near it at all)
Hydrogen Always Screws Up
As usual, hydrogen is a problem.
H
H
With only one valence electron, it doesn't really have a lot of
hope to get to eight. But let's go ahead and make that bond:
H
H
AND WE'RE DONE!
Atoms are trying to get to eight to have a full energy level like the
noble gasses (s2p6). But a full shell in the first level is just 1s2. So
hydrogen doesn't want eight; it wants two.
What About Metals?
They make ionic compounds.
Things Can Get Complicated
This actually turns out to not really be any more difficult than the
ones we've gone over, with a little practice. It will seem easy
soon.
Summary
Share electrons to make a covalent bond
Each element is trying to get to eight electrons around it, except
hydrogen (two).
Shared electrons still belong to their original atoms (so charge is
not affected), but count as being around the atoms for purposes of
the octet rule.
If one bond isn't enough to get you to eight, make more.