Atoms that have an outer shell containing 8 electrons are considered chemically stable.

We often say they fulfill what is called the octet rule. When atoms combine to form compounds, they can give up, accept, or share electrons in order to assemble an octet of electrons in their outer shells.

A covalent bond is formed when atoms share electrons in order to satisfy the octet rule.

For example, a carbon atom holds 4 electrons in its outer shell, but it would be much more stable with 8.

To gain the additional 4 electrons, carbon can share with other atoms and thereby have a completed outer shell. If 1 carbon and 4 hydrogen atoms come together and share their electrons, carbon achieves an octet of electrons.

Note that each hydrogen in this compound achieves 2 electrons in its outer shell.

This is the stable configuration of the first electron shell. An ionic bond forms when two atoms are held together by the attraction between opposite charges.

The reaction between sodium and chlorine atoms is an example of how an ionic bond is formed.

Since sodium has only 1 electron in its third shell, it has a tendency to give up that electron.

Once it does, the remaining outer shell contains 8 electrons.

This loss of an electron gives sodium a slightly positive charge. Chlorine, on the other hand, tends to gain an electron, since its outer shell already has 7.

When chlorine gains an electron, it becomes slightly negative.

The charged sodium and chlorine atoms are called ions.

Ions of opposite charge attract each other and form ionic bonds. In this case, the attraction of sodium and chlorine forms a molecule called sodium chloride, or table salt.