Here's how to figure out the number of electrons in a sulfide ion (S²⁻):

* Neutral Sulfur: A neutral sulfur atom has 16 electrons (the same as its atomic number).

* Gaining Electrons: The superscript "2-" indicates that the sulfur atom has gained two electrons to become an ion.

* Total Electrons: Therefore, the sulfide ion (S²⁻) has 16 + 2 = 18 electrons.