Q.Arrange the following compounds in decreasing (descending) order on the basis of lattice energies: BeF2, AlCl3, LiCl, CaCl2, NaCl.

An ionic (electrovalent) bond forms when one atom transfers one or more of its valence electrons completely to another atom, so that both atoms reach a stable, noble-gas-like electronic configuration — the electron-losing atom becomes a cation and the electron-gaining atom becomes an anion, and the two oppositely charged ions are then held together purely by electrostatic (coulombic) attraction, not by any shared electron pair. It forms readily between an element with a LOW ionisation enthalpy (which gives up an electron easily, e.g. sodium, Na → Na⊕ + e⁻) and an element with a strongly NEGATIVE electron-gain enthalpy (which accepts an electron favourably, e.g. chlorine, Cl + e⁻ → Cl⊖); the same idea extends to multiply-charged ions, e.g. calcium losing two electrons (one to each of two chlorine atoms) to form Ca²⊕(Cl⊖)₂. In the solid state, ionic compounds crystallise as an ordered, repeating lattice of cations and anions, and their true thermodynamic stability comes mainly from the large amount of energy released when this lattice forms — measured as Lattice Enthalpy, the energy required to completely separate one mole of the solid compound back into its gaseous ions (e.g. 788 kJ mol⁻¹ for NaCl) — rather than merely from each ion completing its octet. Lattice enthalpy rises with a HIGHER cationic charge (e.g. AlCl3 > CaCl2 > NaCl) and with a SMALLER cation size (e.g. LiF > NaF > KF), since both factors increase the coulombic attraction holding the lattice's ions together.