The formation of an ionic solid MX form its elements may be formulated by two different paths interrelating different them chemical quantities.

The various terms are
(i)    S = Sublimation energy of the metal

M(s)   →    M(g)

Simnec energy is required for the process (endothermic), its value is reported as positive quantity.
(ii)    D = Dissociation energy of X₂ gas into atoms

   X₂(g)   →   2X(g)

This process is also endothermic and its value is reported as a positive quantity.

(iii)    I = Ionization energy, i.e., energy required for the removal of an electron form an isolated gaseous atom, M. The process is endothermic ad is reported as a positive quantity.

M(g)    →    M+(g) + e

(iv)    E = Electron affinity, i.e., amount of energy released when a natural gaseous atom gains an electron. The process is exothermic and its value is given as a negative quantity.

X(g)+e  →     X-(g)

(v)    U₀ = Lattice energy: it is the amount of energy released when one mole of gaseous positive and negative ions condenses to form an ionic solid.

M+(g) + X-(g)  →   MX(s)

The process is exothermic and its value is always negative.

(vi)    ΔHr = Heat of formation, it is defined as the amount of energy released when one mole of a substance is formed form its element.
M(s) + 1 X₂(g)  →    MX(s)
            2
Energy released in the process and its value is reported as a negative quantity. This is a single step reaction equivalent to sum of all other steps (i) to (v).

Since the two processes are independent of each other, the two can be equated, i.e. hat of formation of MX must be equal to the summation all other terms.

ΔHr  = S+ 1 D + I + E + U₀
                   2
U0 = Δ Hr-S- 1 D-I-E
                        2

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