The English chemist who proposed the early-1800s atomic theory that all matter is made of atoms was John Dalton.
Dalton developed his chemical atomic theory around 1803 and presented a fuller version in A New System of Chemical Philosophy, published from 1808 onward. He argued that each element consists of atoms with characteristic masses, that atoms of different elements combine in simple whole-number ratios, and that chemical reactions rearrange atoms rather than create or destroy them.
His work grew from quantitative chemistry, including measurements showing that elements can combine in multiple fixed ratios. For example, different compounds of nitrogen and oxygen display small whole-number relationships, supporting the idea that matter is built from discrete units. Dalton also published early tables of relative atomic weights.
Some details of Dalton’s model were later revised. Atoms are divisible into subatomic particles, and atoms of the same element can have different masses as isotopes. J. J. Thomson, Ernest Rutherford, and Niels Bohr contributed later discoveries about atomic structure, but none proposed the original broad chemical atomic theory described here.