Why is FeO(s) not formed in stoichiometric composition?

Explanation

In the crystal of FeO, iron to oxygen ratio is slightly less than 1 : 1. This occurs because of the creation of cationic vacancies in the crystal lattice due to missing of some `Fe^(2+)` ions from the crystal. To maintain charge balance in the crystal required number of `Fe^(2+)` ions convert is `Fe^(3+)` ions. For each cationic vacancy two `Fe^(2+)` ions change into `Fe^(3+)` ions.