Why is the sum of the coefficients on the reactant side of a balanced equation does not have to equal the sum of the coefficients on the product side of the equation?

The requirement for a balanced equation is that the numbers of each kind of atoms in the reactants must match the number of each kind of atoms in the products.

The number of atoms in each molecule in the equation is the product of the coefficient and the subscript affixed after each element symbol in the formula to which the coefficient applies.

For example, 2 Na + Cl2 = 2 NaCl.

The coefficients on the left side total 3 but those on the right side total only 2. However, the number of atoms of each of sodium and chlorine total 2 on each side