The law of conservation that states atoms are neither lost nor gained during a chemical reaction is the Law of Conservation of Mass.

This law, first proposed by Antoine Lavoisier in the late 18th century, states that in a closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. Since atoms are the fundamental building blocks of matter, this implies that atoms themselves are not created or destroyed, but merely rearranged during a chemical reaction.