How do you balance this equation Mg + O2 = MgO

2 Mg (s) + O2 (g) → 2 MgO (s)

As magnesium (Mg) has a valency of + 2, and oxygen (O) has a valency of - 2, the ratio would be 1:1, and magnesium oxide would be represented as MgO. It is insoluble in water, hence it has the subscript as a solid, represented by (s). In order to use up the diatomic oxygen (O2), there needs to be two moles of magnesium (2Mg) on the reactant side. This would produce 2 moles of MgO on the product side.