These are a series of chemical formulas for different iron compounds.

Fe3O4 is iron(II, III) oxide, also known as magnetiteFe2O3 is iron(III) oxide, also known as hematiteFeO is iron(II) oxide, also known as wustiteFeCl2 is iron(II) chlorideFeCl3 is iron(III) chlorideFe(NO3)3 is iron(III) nitrate

Each of these compounds has different properties and uses in various industrial and chemical processes.