When reactants react to form products, no atoms are created or destroyed. This principle is called the law of mass conservation. As a result, all chemical reaction equations must be balanced.
In a balanced reaction equation, there is an equal number of atoms for each element in the equation. For the reaction 2 H2 + O2 → 2 H2O, coefficients are added in front of the H2 and H2O molecules to balance the equation. Combined with the subscripts, these coefficients give four hydrogens and two oxygens on the reactant and product side of the equation.