The bond dissociation enthalpy of ${\mathrm{X}}_2 {\mathrm{\Delta H}}_{\text{bond }}^{\circ }$calculated from the given data is _______ kJ mol^–1.
(Nearest integer)

$\begin{array}{l}{\mathrm{M}}^{+}{\mathrm{X}}^{-}\left(\mathrm{s}\right)\to {\mathrm{M}}^{+}\left(\mathrm{g}\right)+{\mathrm{X}}^{-}\left(\mathrm{g}\right){\mathrm{\Delta H}}_{\text{lattice }}^{\circ }=800{\mathrm{kJmol}}^{-1}\\ \mathrm{M}\left(\mathrm{s}\right)\to \mathrm{M}\left(\mathrm{g}\right){\mathrm{\Delta H}}_{\text{sub }}^{\circ }=100{\mathrm{kJmol}}^{-1}\\ \mathrm{M}\left(\mathrm{g}\right)\to {\mathrm{M}}^{+}(\mathrm{g}{)}^{-}+{\mathrm{e}}^{-}(\mathrm{g}){\mathrm{\Delta H}}_{\mathrm{i}}^{\circ }=500{\mathrm{kJmol}}^{-1}\\ \mathrm{X}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\left(\mathrm{g}\right)\to {\mathrm{X}}^{-}\left(\mathrm{g}\right){{\mathrm{\Delta H}}^{\circ }}_{\text{eg }}=-300{\mathrm{kJmol}}^{-1}\\ \mathrm{M}\left(\mathrm{s}\right)+\frac{1}{2}{\mathrm{X}}_2\left(\mathrm{g}\right)\to {\mathrm{M}}^{+}{\mathrm{X}}^{-}\left(\mathrm{s}\right){\mathrm{\Delta H}}_{\mathrm{f}}^{\circ }=-400{\mathrm{kJmol}}^{-1}\end{array}$

[Given : M⁺X^– is a pure ionic compound and X forms a diatomic molecule X₂ is gaseous state]