The chemical formula of iron (II) sulfide is FeS. Because iron sulfide is a chemical compound made from iron bonded to sulfur, the chemical formula is made from a combination of their chemical symbols from the periodic table.
Chemical formulas are based on the properties of the compounds they symbolize. In this case, iron cations are bonded to sulfur anions. A cation is an ion that has fewer electrons than protons, leaving it with a positive charge. An anion is an ion with more electrons than protons, leaving it with a negative charge. The iron has two fewer electrons than it needs to be stable, and the sulfur has two more electrons than it needs, so they bond to create a stable compound. Because there are equal amounts of iron and sulfur, they have assumed subscripts of 1 in the chemical formula. The cation always comes before the anion when naming an ionic compound.