Dalton's Atomic Theory

Dalton's atomic theory, set out in A New System of Chemical Philosophy in 1808, holds that each element consists of identical atoms of a characteristic weight, that compounds form from atoms of different elements combining in fixed small whole-number ratios, and that chemical reactions only rearrange atoms rather than creating or destroying them. It gave chemists their first working method for calculating relative atomic weights and put the existing law of multiple proportions on a physical footing. Later physics refined rather than discarded it: the 1897 identification of the electron and Ernest Rutherford's 1911 nuclear model, built on gold foil scattering results, showed that atoms have internal structure and are not the indivisible units Dalton assumed, and the discovery of isotopes in the 1910s showed that atoms of the same element are not all identical in weight. Dalton's core claim, that matter is composed of discrete atoms which combine with each other in fixed ratios, survived those revisions and remains the working assumption of chemistry.