Final answer:
The coefficient in a chemical equation specifies the number of units (atoms, molecules, or formula units) of reactants and products involved in a chemical reaction, as well as the molar ratios between them.
Step-by-step explanation:
The coefficient in a chemical equation is a number placed immediately to the left of a chemical formula that indicates the relative number of units (atoms, molecules, or formula units) involved in the reaction. For example, in the equation 2H2 + O2 → 2H2O, the coefficient '2' before H2 indicates that two molecules of hydrogen are used, and the same coefficient before H2O indicates that two molecules of water are produced. It's important to note that a coefficient of '1' is often omitted because it is understood.
In a balanced chemical reaction, coefficients also represent the molar ratio between reactants and products and are used to determine how many moles of each substance participate in the reaction.
Unlike subscripts, which are part of chemical formulas and indicate the number of atoms of an element in a molecule, coefficients apply to the entire molecule or formula unit. Changing a subscript would change the actual compound, while changing a coefficient adjusts the quantity of that compound in the balanced equation.