Which element would release the most energy while adding an electron to a neutral atom in the gas phase?

Adding an electron to a neutral gaseous atom releases energy when the atom has a strong electron affinity. Electron affinity generally increases across a period, and halogens are especially likely to gain an electron because doing so completes their outer shell. Among Na, Al, Br, and S, bromine is the halogen with the greatest tendency to release energy when gaining an electron, so the correct choice is Br.

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