Discussion
Which one of the following is the correct decreasing order for the magnitude of Electron Gain Enthalpy (\(\Delta E_{eg}H\)) for the elements given below?
Answer: B
The correct decreasing order is Cl > Br > I > At.
Electron gain enthalpy becomes less negative down group 17 because the added electron enters a shell further from the nucleus and is held less firmly. Chlorine releases about 349 kJ/mol on gaining an electron, bromine about 325, iodine about 295, and astatine less still.
Chlorine heads the group, rather than fluorine, because fluorine's small 2p shell is so crowded that electron repulsion reduces its value; fluorine is not in this list, so chlorine is simply first. The options that put bromine, iodine or astatine ahead of chlorine reverse the down-the-group trend.
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