Iron pyrite (fool's gold) is iron (ii) sulphide. What is its formula?

Iron pyrite, commonly known as fool's gold, has the chemical composition of iron (II) sulfide. To derive its formula, we need to consider the following:

1. **Iron (II) Ion**: The iron in pyrite is in the +2 oxidation state, which is denoted as Fe²⁺.

2. **Sulfide Ion**: The sulfide ion is derived from sulfur and has a charge of -2, denoted as S²⁻.

To form a neutral compound, the total positive charge must equal the total negative charge. In this case:

- One Fe²⁺ ion provides a charge of +2.
- One S²⁻ ion provides a charge of -2.

Since one iron (II) ion balances the charge of one sulfide ion, the formula for iron pyrite is:

\[
\text{FeS}
\]

This indicates that one iron ion and one sulfide ion combine to form iron pyrite (fool's gold).