The bond dissociation energies of X2, Y2, and XY are in the ratio of 1:0.5:1. ΔH for the formation of XY is -600 kJ mol⁻¹. The bond dissociation energy of X2 will be:

ΔH for the formation of XY is -600 kJ mol⁻¹.
Let the bond dissociation energies of X2, Y2, and XY be x, 0.5x, and x respectively.
The formation of XY can be represented as:
1/2X2 + 1/2Y2 → XY
ΔH = [Bond dissociation energy of 1/2X2 + Bond dissociation energy of 1/2Y2] - [Bond dissociation energy of XY]
ΔH = [1/2x + 1/2(0.5x)] - x
-600 = 0.25x - x
-600 = -0.75x
x = 800 kJ mol⁻¹
Therefore, the bond dissociation energy of X2 is 800 kJ mol⁻¹.