Forming ions

Ions in reactions

Ions are charged particles formed when atoms, or groups of atoms, lose or gain electrons to obtain full outer shells. Ions form when a metal reacts with a non-metal. In the reaction:

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

For example, sodium reacts with chlorine to form sodium chloride. The transfer of electrons can be modelled using dot and cross diagrams. The electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses. The dots and crosses do not need to be coloured.

The outer electron from a sodium atom transfers to the outer shell of a chlorine atomAn electron transfers from the outer shell of a sodium atom to the outer shell of a chlorine atom

Sodium ions and chloride ions form in the reaction.

Structures of a sodium ion and a choride ion.Sodium chloride contains sodium ions and chloride ions

Positive ions

A magnesium atom next to a magnesium ion. The ion shows that the atom has lost its two outer electronsA magnesium atom loses two electrons to form a magnesium ion.

Metal atoms lose electrons when they form ions. These are positively charged because they have fewer electrons in their shells than protons in their nuclei.

For elements in group numbers 1, 2 and 3 (IUPAC groups 1, 2 and 13) the number of electrons lost is the same as the old group number.

IUPAC group number1213
Old group number123
Charge on ions+2+3+
Example ionNa+Mg2+Al3+

Negative ions

Non-metal atoms gain electrons when they form ions. These are negatively charged because they have more electrons than protons.

How an oxygen atom becomes an oxygen ion when it gains two electronsAn oxygen atom gains two electrons to form an oxide ion

For elements in old group numbers 6 and 7 (IUPAC groups 16 and 17) the number of electrons gained is equal to (8 - old group number).

IUPAC group number1617
Old group number67
Charge on ions2--
Example ionO2-Cl-