Bond enthalpies of ${\mathrm{A}}_2,{ \mathrm{B}}_2$ and $\mathrm{AB}$ are in the ratio $2: 1: 2.$ If bond enthalpy of formation of $\mathrm{AB}$ is $-100 \mathrm{kJ} {\mathrm{mol}}^{-1}.$ The bond enthalpy of ${\mathrm{B}}_2$ is