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, because they have 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
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 formed have full outer shells
  • for groups 6 and 7, the number of electrons gained is equal to: 8 - 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

Examples of groups and ion charges

GroupElementIon chargeIon symbol

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 gives the number of protons
  • mass number gives the total number of protons + neutrons
  • the number of electrons = the number of protons (if atom is neutral)
  • positive ions have lost electrons
  • negative ions have gained electrons
Ion nameIon symbolAtomic no.Mass no.ProtonsNeutronsElectrons

Barium is a metal in group 2 of the periodic table. What happens to the electronic structure of a barium atom when it forms ions?

Barium loses the two electrons in its outer shell to form Ba2+ ions.