Forming ions

An ion is an atom or group of atoms with a positive or negative charge. Ions form when atoms lose or gain electrons to obtain a full outer shell:

• metal atoms lose electrons to form positively charged ions
• non-metal atoms gain electrons to form negatively charged ions

Forming positive ions

Metal atoms lose electrons from their outer shell when they form ions:

• the ions formed are positive, with more protons than electrons
• the ions formed have full outer shells

For elements in groups 1, 2 and 3, the number of electrons lost is the same as the group number.

A sodium atom loses one electron to form a sodium ion

Forming negative ions

The outer shell of non-metal atoms gains electrons when they form ions:

• the ions formed are negative, because they have more electrons than protons
• the ions formed have full outer shells

For groups 6 and 7, the number of electrons gained is equal to (8 - group number).

An oxygen atom gains two electrons to form an oxide ion

Example of ion charges and groups

The numbers of protons, neutrons and electrons in an ion can be calculated from its atomic number, mass number and ionic charge.

Remember that:

• atomic number = protons
• mass number = protons + neutrons
• electrons = protons (if atom is neutral)
• positive ions have lost electrons
• negative ions have gained electrons

Some examples are shown in the table below: