Answer: Br
Explanation:
\(Br:35:[Ar]3d^{10}4s^24p^5\)
\(Br^-:36:[Ar]3d^{10}4s^24p^6:[Kr]\)
\(Na:11:[Ne]3s^1\)
\(Na^-:12:[Ne]3s^2\)
\(Al:13:[Ne]3s^23p^1\)
\(Al^-:14:[Ne]3s^23p^2\)
\(S:16:[Ne]3s^23p^4\)
\(S^-:17:[Ne]3s^23p^5\)
As Bromine is short of only 1 electron to attain noble gas configuration, it will release energy as more stable species have less energy. Thus bromine would release most energy while adding an an electron to a neutral atom in the gas phase.
All other species Na, Al and S would not attain stable configuration on addition of electron and hence no energy would be released.
