Here's why:

* Sulfur's natural state: Sulfur has 16 electrons in its neutral state.

* Sulfur's electron configuration: The electron configuration of sulfur is 2-8-6. This means it has 6 electrons in its outermost shell (valence shell).

* Octet Rule: Atoms tend to gain or lose electrons to achieve a stable octet (8 electrons) in their outermost shell.

* Sulfide ion formation: Sulfur gains two electrons to achieve a full octet (2-8-8) and becomes a sulfide ion (S²⁻).