The atomic theory formulated by the British chemist John Dalton proposes that atoms are indivisible particles comprising all matter; atoms of the same element share identical properties, including shape, size and mass; atoms of an element are unique and vary from the atoms of different elements; atoms are the fundamental units of a chemical reaction; atoms are conserved and can neither be created nor destroyed; and atoms combine in simple, fixed proportions to produce compound atoms, known as molecules. Dalton is commonly referred to as the father of the modern atomic theory.
The atomic theory originated in ancient Greece when the philosopher Democritus first suggested that matter consists of indivisible particles, which he named "atomos." Advancements in the scientific method lead to the development and refinement of the atomic theory from its initial conception. Prominent scientists, such as Dalton, Thomson, Chadwick, Moseley and Rutherford, made major contributions to the modern atomic theory.
Dalton published his atomic theory in 1808, which was compiled from the results of the various experiments he conducted in studying the atom. The conservation law and the laws of chemical combination were also used by Dalton to support his theory. However, some of Dalton's assumptions have been proven wrong. The atom, which he believed to be indivisible, contains subatomic particles. The existence of isotopes and isobars also demonstrates that atoms of the same element can have variable masses and atoms of different elements can have the same atomic masses.