The atom
Oxygen gaining electrons

Oxygen needs two electrons to fill the outer energy level. It will get two electrons from another atom.

Notice how the oxygen atom takes two electrons to look like neon. We can tell that it is an oxygen atom by the number of protons in its nucleus. The imbalance of protons and electrons now creates a charge of -2 on the atom.

Atoms with a charge are known as IONS. More precisely atoms with a positive charge are known as cations while atoms with a negative charge are known as anions. Oxygen, above, forms an anion.