Here's why:

* Sulfur's electron configuration: Sulfur has 16 electrons. Its electron configuration is [Ne] 3s² 3p⁴. 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 configuration with 8 electrons in their outermost shell (like noble gases).

* Sulfur's tendency: Sulfur is two electrons short of a full outer shell. To achieve this stability, it gains two electrons, resulting in a -2 charge.

Therefore, the sulfur ion is written as S²⁻.