What principle states that the number of atoms remains equal on both sides of a chemical equation?

The principle that states the number of atoms remains equal on both sides of a chemical equation is the Conservation of Mass. This fundamental concept in chemistry posits that mass cannot be created or destroyed in a closed system during any chemical reaction. Consequently, the mass of the reactants must equal the mass of the products, leading to the conclusion that the number of each type of atom must also be equal before and after the reaction. This is why, when balancing a chemical equation, chemists ensure that there are equal numbers of each type of atom on both sides.

The other options focus on different aspects of chemistry. The Law of Conservation of Energy pertains to the idea that energy cannot be created or destroyed, only transformed, which is distinct from mass conservation. Atomic Theory outlines the nature and behavior of atoms but does not specifically address the conservation aspect in chemical reactions. The Mass Action Principle relates to the rates of reactions and concentrations of reactants and products at equilibrium, but it does not focus on the conservation of atoms.