The bond energies of $A_2,B_{2}and\text{\hspace{0.17em}\hspace{0.17em}}AB$ are in the ratio   2:1:2.If the enthalpy of formation of AB from $A_{2}and{B}_2$  is -150 KJ/mole, then the bond dissociation energy( in KJ/mole) of  $A_2$ is________