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 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.

How a sodium atom becomes a sodium ion when it loses an electronA sodium atom loses one electron to form a sodium ion

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).

How an oxygen atom becomes an oxygen ion when it gains two electronsAn oxygen atom gains two electrons to form an oxide ion
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Ions are formed by the transfer of electrons.

Example of ion charges and groups

GroupElementIon chargeIon symbol
1Na+Na+
2Mg2+Mg2+
6O2-O2-
7Cl-Cl-

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.

Question

Iodine is in group 7. What is the charge on its ions, and is the charge positive or negative?

The charge is negative, since iodine is a non-metal. The charge on the ion is (8 - 7) = 1.