Sulfur (S) becomes isoelectronic with argon (Ar) when it gains two electrons.

Here's why:

* Sulfur's electron configuration: [Ne] 3s² 3p⁴

* Argon's electron configuration: [Ne] 3s² 3p⁶

By gaining two electrons, sulfur fills its 3p subshell, achieving the same electron configuration as argon. Isoelectronic species have the same number of electrons, and therefore, similar chemical properties.