A chemical equation in which both reactant and product have an equal number of atoms of each element is called a balanced chemical equation. In such an equation, the number of atoms of each element on the reactant side is equal to the number of atoms of the same element on the product side. This stoichiometric principle ensures that the law of conservation of mass is satisfied, stating that matter cannot be created or destroyed in a chemical reaction.

To balance a chemical equation, coefficients are added to the reactants and products to adjust their stoichiometry. These coefficients represent the relative amounts or moles of each substance involved in the reaction. Balancing an equation involves changing these coefficients while maintaining the chemical identity of the reactants and products.

For example, consider the unbalanced chemical equation for the combustion of methane:

$$CH_4 + O_2 → CO_2 + H_2O$$

To balance this equation, we add coefficients to the reactants and products as follows:

$$CH_4 + 2O_2 → CO_2 + 2H_2O$$

Now, both the reactant and product sides of the equation have an equal number of carbon, hydrogen, and oxygen atoms:

Reactants: 1 C, 4 H, 2 O

Products: 1 C, 4 H, 2 O

By balancing the equation, we ensure that there is no net change in the number of atoms of each element during the reaction, upholding the law of conservation of mass.