125. Discuss the factors which favour the formation of ionic bond.

Following factors influence the formation of ionic bonds:

(i) Low ionisation enthalpy: One of the atoms forming cation should have low ionisation enthalpy. Alkali metals (Group 1) and alkaline earth metals (Group 2) having low values of ionisation enthalpy from their cations readily.

(ii) Very high negative electron gain enthalpy: The other atom-forming anion should have very high negative electron gain enthalpy. Elements of group 16 and group 17 (halogens) having very high negative electron gain enthalpy from their anions readily.

M(g) → M⁺(g) + e– : Ionisation enthalpy
X(g) + e^– → X ^–(g) : Electron gain enthalpy
M⁺(g) + X ^–(g)→ MX(s)

Clearly, ionic bonds will be formed more easily between elements with comparatively low ionisation enthalpies and elements with comparatively very high negative electron gain enthalpies.