 Perrin originally proposed the name Avogadro 's number ( N ) to refer to the number of molecules in one gram @-@ molecule of oxygen ( exactly 32g of oxygen , according to the definitions of the period ) , and this term is still widely used , especially in introductory works . The change in name to Avogadro constant ( NA ) came with the introduction of the mole as a base unit in the International System of Units ( SI ) in 1971 , which recognized amount of substance as an independent dimension of measurement . With this recognition , the Avogadro constant was no longer a pure number , but had a unit of measurement , the reciprocal mole ( mol − 1 ) . 
