Low Ionisation Enthalpy : The atom going to form + ve ion should have low value of I.E. e.g. Na, K, Mg, Ca.
(ii) High Electron gain enthalpy : The atom which
is going to form -ve ion should have high value of electron gain enthalpy.
(iii) Lattice Enthalpy: The higher the value of lattice enthalpy higher will be stability of the compound and higher will be the case of formation.
${{M}^{+}}$ (g) + ${{X}^{-}}$(g) —> MX (s) + Lattice 1 mole energy