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Considering the given elements \(A{l^ + },{\rm{ }}C{l^ + },{\rm{ }}{I^ + },{\rm{ }}N{a^ + },{\rm{ }}R{b^ + }\) with \(1 + \) charge. In an exothermic reaction, excess energy is given out. Observing the elements,

\(A{l^ + }\)will have filled s-subshell.

\(N{a^ + }\)will have filled p-subshell.

\(R{b^ + }\)will have filled p-subshell.

In the case of Cl, the electron will fall into the lowest 3p orbital. In the case of iodine, the electron will fall into a 5p orbital.

Observing the data mentioned above, we can conclude that Cl-atom will have the most exothermic reaction.

When an electron moves from a higher energy state to a lower energy state energy is released. This energy is released in form of heat is called exothermic energy.