What term describes a number that indicates the quantity of a chemical formula in a reaction?

The term that indicates the quantity of a chemical formula in a reaction is "coefficient." Coefficients are placed in front of the chemical formulas in a balanced equation to denote how many molecules or moles of each reactant and product participate in the reaction. For instance, in the reaction 2H₂ + O₂ → 2H₂O, the coefficient '2' in front of H₂ indicates that two molecules of hydrogen are involved in the reaction.

In contrast, subscripts refer to the number of atoms of an element within a molecule but do not indicate the quantity of molecules or the ratio of reactants and products in a chemical reaction. Elements represent the individual types of atoms like hydrogen or oxygen without context for quantity, and a molecule is a distinct chemical structure formed by atoms bonded together. Thus, coefficients are essential for capturing the stoichiometric relationships in a chemical reaction, allowing for proper interpretation and calculations based on the balanced equation.