Down the group, the size of halogen increases. The valence orbital used for bonding becomes larger and more diffused. Th

[answerhappygod]

Down the group, the size of halogen increases. The valenceorbital used for bonding becomes larger and more diffused. There isless effective orbital overlap between the 1s orbital of hydrogenand the valence p orbital of the halogen. Hence, H-X bond strengthdecreases down the group as shown by the decrease in bond energyfrom +431 KJ mol⁻¹ in H-Cl to +299 KJ mol⁻¹ in H-I. Since H-X bondis more easily broken down the group, thermal stability of thehalogen halide decreases.