# How many oxygen atoms are present on the reactant side of the chemical equation 4Fe + 3O_2 -> 2Fe_2O?

$4 F e \left(s\right) + 3 {O}_{2} \left(g\right) \rightarrow 2 F {e}_{2} {O}_{3}$
Clearly, there are $6$ oxygen atoms on the left hand side of the equation; and, equally clearly, there are $6$ oxygen atoms on the right hand side of the equation, as indeed there must be in a stoichiometrically balanced equation.