Forming negative and positive ions

Forming negative ions (anions)

Atoms gain electrons in their outer shell when they form negative ions, called anions. These ions are negative because they contain more electrons than protons.

Oxide ionAn oxygen atom gains two electrons to form an oxide anion.

Forming positive ions (cations)

Atoms lose electrons from their outer shell when they form positive ions, called cations. These ions are positive because they contain more protons than electrons.

Sodium ionA sodium atom loses one electron to form a sodium cation.

Examples of ion charges and groups

GroupElement symbolIon ChargeIon Symbol
1Na+Na+
2Mg2+Mg2+
6O2-O2-
7Cl-Cl-
curriculum-key-fact
When writing the charge on the ion, remember to put the number before the positive or negative symbol (2+). Just write + or – if the charge is 1+ or 1-.