Covalent Bonding

Consider the ChlorineA single, neutral chlorine atom has 7 valence electrons: [Ne]3s23p5

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 SolutionLike 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 NaBecause 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-

Of course, that chlorine could take two from another chlorine, but that makes that chlorine short three.

THAT WAY LIES MADNESS!

Cl+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

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!

Cl

Bond. Covalent BondCovalent 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:

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').

OWe're still sad. :(

O OYou 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 UpDrawing bonds as lines is a little more simple visually, and also just moves the atoms farther apart.

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.

O

Mix and MatchThere's no reason we need to limit ourselves to two atoms, or to two of the same type of atom:

Cl

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.

O F

Cl O F(Don't breathe near this molecule. In fact, just don't be near it at all)

Hydrogen Always Screws UpAs usual, hydrogen is a problem.

HWith 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 HAND 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.

SummaryShare 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.