When sulfur reacts with a metal to make an ionic compound, it forms a sulfide ion.

Here's why:

* Sulfur's electronegativity: Sulfur is more electronegative than most metals. This means it has a stronger attraction for electrons.

* Ionic bond formation: When sulfur reacts with a metal, it gains two electrons to achieve a stable octet configuration, forming a sulfide ion (S²⁻). The metal atom loses electrons to form a positively charged cation.

* Example: Sodium (Na) reacts with sulfur (S) to form sodium sulfide (Na₂S). Sodium loses one electron to become Na⁺, and two sodium ions combine with one sulfide ion (S²⁻).

So, the general formula for an ionic compound formed between a metal and sulfur is Metal sulfide.