The atomic number of an element doesn't change when it gains or loses electrons. Therefore, the atomic number of S-2 (sulfide ion) is the same as the atomic number of sulfur (S), which is 16.

Here's why:

* Atomic number: The number of protons in the nucleus of an atom. This number defines the element.

* S-2: This represents a sulfur atom that has gained two electrons, giving it a -2 charge.

* Protons vs. Electrons: The number of protons determines the atomic number. Adding or removing electrons only changes the charge of the atom, not its identity.