Lavoisier and Mendeleev on the elements.

Abstract

Lavoisier defined an element as a chemical substance that cannot be decomposed using current analytical methods. Mendeleev saw an element as a substance composed of atoms of the same atomic weight. These `definitions' do quite different things: Lavoisier's distinguishes the elements from the compounds,so that the elements may form the basis of a compositional nomenclature; Mendeleev's offers a criterion of sameness and difference for elemental substances, while Lavoisier's does not. In this paper I explore the historical and theoretical background to each proposal. Lavoisier's and Mendeleev's explicit conceptions of elementhood differed from each other, and from the official IUPAC definitionof `element' of the 1920s. However, Lavoisier and Mendeleev both subscribed to – and employed – a deeper notion of a chemical element as the component of compound substances that (i) can survive chemical change, and (ii) explains the chemical behaviour of its compounds.