Atomic theory states that all matter in the universe is composed of atoms. These atoms make up molecules that combine to create compounds. This was originally theorized in the fifth century B.C. by the Greek philosopher Democritus.
Atomic theory has been refined since Democritus's time, taking its greatest leap between 1789 and 1803. During that time, three primary laws of atomic theory were developed. These include the Law of Conservation of Mass, the Law of Definite Proportions and the Law of Multiple Proportions.
The first is credited to Antoine Lavoisier, and states that the mass involved in any chemical reaction remains constant. Joseph Louis Proust proved the second law in 1799, which states that regardless of their original source or quantity, if a compound is reduced to its base elements, the mass of those elements remain proportionate. Finally, in 1803, John Dalton completed the trio of laws by studying and expanding on previous research. His contribution was the only one to be arrived at through the scientific method, which includes performing experiments and examining the resulting information in an empirical fashion. Though science has discovered far more about the nature of the atom since then, these original laws have remained constant in the scientific community.