# Iron pyrite (fool's gold) is iron (lI) sulfide. What is its formula?

Iron pyrites is $\text{iron persulfide}$, $F e {S}_{2}$, with a formal $F e \left(I I +\right)$ oxidation state.
The sulfur binds as a $\text{persulfide}$, i.e. ""^(-)S-S^-; think $\text{peroxide}$, ${O}_{2}^{2 -}$. The formal oxidation state of $S$ here is $- I$.