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

* Start with the neutral sulfur atom: Sulfur (S) has 16 electrons (its atomic number is 16).

* Consider the charge: The sulfide ion (S²⁻) has a charge of -2. This means it has gained two extra electrons.

* Add the extra electrons: 16 (original electrons) + 2 (gained electrons) = 18 electrons.

Therefore, a sulfide ion (S²⁻) has 18 electrons.