The coefficients in a chemical equation represent the

The coefficients in a chemical equation represent the relative number of moles of each reactant and product that interven in the chemical reaction.

Explanation:

The coefficients are the numbers that you put in front of each chemical formula that represents the reactants and products in the chemical equation. They indicate the mole ratio in which the elements or compounds react to form the products, as per the chemical equation.

See an example:

Word equation: hydrogen and oxygen produce water

Chemical (skeleton) equation: H₂ (g) + O₂ (g) → H₂O (g)

This equation is not balanced: the number of atoms of oxygenin the reactant side is 2 while the number of atoms of oxygen isn the product side is 1. In order to balance the equation you need to add some coefficients.

When no coefficients are shown it is understood that the coefficient is 1.

Balanced chemical equation: 2H₂ (g) + O₂ (g) → 2H₂O (g)

The coefficients 2 in front of H₂ and 1 (understood) in front of O₂, in the reactant side, and 2 in front of H₂O, in the product side, balance the equation.

Those coefficients mean that the 2 molecules (or mole of molecules) of H₂ react with 1 molecule (or mole of molecules) of O₂ to form 2 molecules (or moles) of H₂O (product side).

That is the mole ratio: 2 H₂ : 1 O₂ : 2 H₂O.

Notice that, in spite of the aboslute numbers may change, the mole ratio is unique for any chemical reaction. For example 4 : 2 : 4 is the same ratio that 2 : 1 : 2, or 8 : 4 : 8, but the most common practice is to use the most simple form of the ratio, i. e. 2: 1: 2.