To balance the synthesis reaction between magnesium Mg and oxygen O2 , we need to make sure that the number of atoms for each element is equal on both sides of the equation. The unbalanced equation is:Mg + O2 MgONow, we balance the equation using the least whole number coefficients:2Mg + O2 2MgOIn this balanced equation, there are 2 magnesium atoms and 2 oxygen atoms on both sides of the equation, satisfying the law of conservation of mass.