To find the coefficient of a chemical compound, you must balance the elements involved in it. For example, water: H 2 O. It just so happens that hydrogen (H) and oxygen (O) are both diatomic molecules, thus we have H 2 and O 2. To form water, one of the O atoms breaks off from the O 2 molecule and react with the H 2 compound to form H 2 O. But, there is one oxygen atom left. It reacts with another H 2 molecule. Since it took two of each atom to balance the compound, we put the coefficient 2 in front of H 2 O: 2 H 2 O. The total reaction is thus 2 H 2 + O 2 → 2 H 2 O.