The idea that matter consists of indivisible particles was not new — in the 5th century BCE Leucippus and Democritus introduced the atom as a philosophical concept, and Lucretius carried it to Roman readers. For two millennia, however, the atom remained an opinion; it was not tied to experiment. By the late 18th century Antoine Lavoisier had established the conservation of mass and Joseph Proust the law of definite proportions, and chemistry was beginning to become quantitative.

A Quaker meteorologist teaching in Manchester, John Dalton, while studying gases and atmospheric composition, noticed that elements combine in simple whole-number ratios. In a 1803 notebook he drew the first table of atomic weights, taking hydrogen as 1 and weighing other elements against it. The 1808 A New System of Chemical Philosophy made five claims explicit — each element consists of its own atoms, atoms of the same element are identical, atoms of different elements have different masses, compounds are atoms combined in whole-number ratios, and chemical reactions are rearrangements of atoms.

Many of Dalton's numerical values were wrong (he assumed water was HO and assigned atomic weights accordingly), and his circle-in-symbol notation was displaced by modern chemical symbols. But the core claim endured: the structure of matter is not continuous but discrete; these units can be weighed and counted. The rest of the 19th century was spent filling in the framework — Avogadro's hypothesis, Cannizzaro's atomic weights, Mendeleev's periodic table.

The atom itself did not stay indivisible. In 1897 J. J. Thomson found the electron, in 1911 Rutherford the nucleus, in 1913 Bohr quantised orbits; in the 20th century quantum mechanics rewrote the interior. Still, 1803 is treated as the reckoning year of modern chemistry as a discipline: the atom is no longer a metaphor but a unit of measurement.