Molecules have covalent bonds

Atoms usually don't come one by one. They like to attach to each other. Sometimes they form large crystal structures... ... and sometimes they form smaller units called molecules. Why do they attach to each other to form molecules?

It has to do with the electrons, and more specifically, the electrons in the outermost electron shell, the valence electrons. Atoms that have close to eight electrons in their valence shell want to get more electrons, to reach the number eight. If there's another atom willing to give up some of its electrons, we get a negative and a positive ion. These ions attract each other, and form large crystal structures with billions of ions. But that only happens when one atom wants more electrons, and another wants to get rid of its electrons.

What if two atoms both want to get more electrons, but neither is willing to give them up? There is a simple solution. They can co-operate. Both of these atoms want to get an extra electron. But neither wants to give any away.

But they can both offer one of their electrons to the other. Let's count the outer electrons of each atom now: 1, 2, 3, 4, 5, 6, 7... 8. This atom has eight electrons in its outer shell, just as it wants. And the other one: 1, 2, 3, 4, 5, 6, 7..

8. This atom also has eight outer electrons. By each sharing one of their electrons, they can both in effect, get one more electron, without giving any away. As long as the atoms stay close to each other, this shared "electron pair" belongs to both atoms. The shared electrons have created a chemical bond between the atoms, shown as a line between them.

The atoms have now formed - a molecule. The bonds between atoms in a molecule are called covalent bonds. Since these atoms only needed one more electron, they can only form one covalent bond. Here's an atom with six electrons in its outer shell. To reach the number eight, it needs to get two more electrons.

One way is to form covalent bonds with two different atoms. If we count the electrons again, we have: Two electrons in this bond. Two in this bond. And four electrons that are non-bonded. A total of eight.

The atom lacked two electrons, so it could make two bonds. But there is another way. If the atoms share two electrons each instead of one... They form a double bond. The double bond consists of two electron pairs, and is stronger than a single bond.

If you start out with five valence electrons, how many bonds do you want to form? Three single bonds. Or, a triple bond. That is even stronger than the double bond. And there you have it.

Atoms form molecules by sharing electrons with each other, in covalent bonds. That way, they can increase the number of electrons in their outer shells, without stealing any electrons away from each other. For each bond, the atom gains one electron. Double bonds and triple bonds are also possible. The goal is to reach eight valence electrons.