Final answer:
Bond dissociation enthalpy decreases down a group because of the increase in atomic size.
Step-by-step explanation:
Bond dissociation enthalpy is the energy required to break a bond in one mole of gaseous molecules. Down a group in the periodic table, the bond dissociation enthalpy generally decreases. This is because as you move down the group, the atomic size increases which leads to a decrease in the strength of the bond. For example, in the halogen group, fluorine has the highest bond dissociation enthalpy while iodine has the lowest.
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