Ionic bonding

Key Definition Ionic bonding is the strong net electrostatic attraction between oppositely charged ions arranged in a giant three-dimensional lattice.

Ions form when metal atoms lose electrons to become cations and non-metal atoms gain electrons to become anions, with both species achieving a noble gas electronic configuration.

The existence of ions is supported by three independent lines of evidence:

• The high melting points of ionic compounds and their electrical conductivity when molten
• Electron density maps showing closed contours (lines of equal electron density) around each ion
• The migration of coloured ions to the oppositely charged electrodes during electrolysis

The strength of ionic bonding increases with greater ionic charge and smaller ionic radius, because both factors increase the electrostatic attraction between the ions.

Ionic radius increases down a group as additional electron shells are added, and decreases across a set of isoelectronic ions as nuclear charge increases while the electron count stays the same.

Polarisation

Key Definition Polarisation describes the distortion of an anion’s electron cloud by a neighbouring cation, introducing partial covalent character into the bond.

Cation polarising power increases with higher charge and smaller radius, while anion polarisability increases with higher charge and larger radius.

The exam focus across this subtopic is precision of language:

• The bond is the electrostatic attraction, not the electron transfer
• The lattice is described as giant and three-dimensional
• Trends in ionic radius must always be justified by either a changing number of electron shells or a changing nuclear charge at constant electron count
• Polarisation arguments must always identify the polarising cation and the polarisable anion separately, stating the relevant size and charge factors for each