Iron(III) slufide's formula can be deduced by starting t=with the oxidation number for the iron, which is 3 in this case. That means iron has given up three electrons, so it can be written as Fe3+. Sulfur is in group 16 in the periodic table and tends to take 2 electrons in order to fill it's outer electron orbital. Thus, it would be represented as S2-. For these two elements to combine to form a neutral product, they must do so in a way that makes the resulting charge 0, or neutral. You can't use fractions of an element, so the simplest combination of Fe and S would be 2 irons and 3 sulfurs. The six electrons donated by the 2 Fe atoms would match the six electrons consumed by the 3 sulfurs.
Fe2S3
This produces a neutral molecule where the six electrons involved are shared among the elements, but who spend more time on the sulfur atoms since they help sulfur achieve a full outer shell, a preferred low energy state.
Bob
