Dalton's Atomic Theory
The 1808 model proposing that elements consist of indivisible atoms that are identical within an element and combine in fixed whole-number ratios.
Unit
Dimensionless
In depth
Its five postulates explained conservation of mass, definite proportions and multiple proportions in a single framework. Two postulates have since been amended: atoms are divisible into subatomic particles, and atoms of the same element can differ in mass as isotopes. The core insight — that matter is particulate — survives intact.
Examples in the real world
Dalton used it to predict that carbon and oxygen would form compounds in simple ratios, later confirmed as CO and CO₂.