F, Cl, and Br are strong oxidising agents because they have a high electron affinity value and a high ability to accept electrons by oxidising other elements.

Low ionisation enthalpies allow an element to donate an electron to another element, reducing that other element, and acting as a powerful reducing agent.

Reaction is,

$$\begin{gathered} {{\mathrm{s}}^{ }}_{\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}⟶{\mathrm{s}}^{-} \\ {\mathrm{s}}^{-}+{\mathrm{e}}^{-}⟶{\mathrm{s}}^{2-} \end{gathered}$$

The formation of ${\mathrm{S}}^2-\left(\mathrm{g}\right)$ is an endothermic process.