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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

Learn more on ionic bonding in this podcast.

Listen to the full series on ����ý Sounds.

Forming positive ions

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

• the ions are positive, because they have more protons than electrons
• the ions formed have full outer shells
• the ions have the electronic structure of a noble gas (group 0 element), with a full outer shell

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

Forming negative ions

The outer shells of non-metal atoms gain electrons when they form ions:

• the ions formed are negative, because they have more electrons than protons
• the ions have the electronic structure of a noble gas (group 0 element), with a full outer shell

For elements in groups 6 and 7, the charge on the ion is equal to (8 minus group number).

Example of ion charges and groups

Worked example

Sulfur is in group 6 of the periodic table. What is the charge on its ions, and is the charge positive or negative?

The charge is negative, since sulfur is a non-metal. The charge on the ion is (8 - 6) = 2.