Why you can't you write the formula of the ionic compound that forms between Fe and S?

Answer 1

You actually can write the ionic formula for Iron Sulfide! The most common form is simply FeS but it can also form other minerals. Pyrite (fool's gold), for example, is FeS₂. Iron forms these different sulfide compounds because it can have a different oxidation number, or a different number of electrons. Iron (II) sulfide has iron with a 2+ charge while Iron (IV) sulfide has a 4+ charge on the iron atom. Hope this helps :P