Final answer:
Bond distance increases and bond dissociation enthalpy decreases as we go down group 7 in the periodic table, due to an increased number of electron shells and lesser orbital overlap.
Step-by-step explanation:
As we go down group 7 in the periodic table, the bond distance between the atoms in a molecule typically increases due to the addition of electron shells. This increase in bond length results in a lesser overlap between the 1s orbital of hydrogen and the valence orbital of the halogen atom, leading to a decrease in the bond dissociation enthalpy. For example, the bond dissociation energies for carbon-halogen bonds decrease in the order: C-F (439 kJ/mol) > C-Cl (330 kJ/mol) > C-Br (275 kJ/mol). This trend indicates that as we move down the group, from fluorine to bromine, the bonds become weaker, correlating with the increase in bond length and decrease in enthalpy.