Atomic theory is the idea that matter is made up of little units called atoms. When the ancient Greek philosopher Democritus came up with the idea in the 5th century BC, is was originally meant to refer to indivisible units. As of 1897, the British scientist J.J. Thomson discovered that atoms are in fact made up of smaller particles. Today, this theory refers to matter being made up of units that are indivisible only some of the time. Exceptions include plasmas, such as fire, other ionic arrangements, such as those found in the body, radioactive materials, and many more.
Even though atomic theory today is a familiar cornerstone of modern science, like germ theory or evolution, throughout most of human history, people believed that matter was probably continuous and could be broken down into arbitrarily small quantities. It wasn't until 1803, or possibly a bit before, that the English chemist John Dalton revived the old idea and used it to solve various problems that chemists were grappling with at the time. Rather than any one experiment leading to the idea, it emerged from analysis of multiple experiments involving the properties of gases and chemical reactions. His theory was popularized and confirmed experimentally over the course of the early 19th century.
Dalton's atomic theory had five main points:
- All matter consists of minuscule particles called atoms.
- All atoms of a given element are identical to each other.
- All atoms of a given element are different than those of other elements.
- Atoms of one element combine with other elements to create compounds. They always combine in equal amounts.
- Atoms cannot be created, divided, nor destroyed.
Most of the above is still accepted by scientists today, except for a few points. First, the discovery of nuclear fusion/fission and radioactivity prompted revision of point #2. Isotopes prove that atoms of the same element can actually have small differences due to a different number of neutrons. Also, the existence of ions with varying numbers of electrons also contradicts this point.
The fifth point is also invalidated by nuclear physics, since atoms can indeed by destroyed in nuclear chain reactions. The second item of point #4 is also quite incorrect, as, for instance, water is H2O, not HO. His insistence that atoms combine in equal amounts to create compounds held back acceptance of his theory for years. Regardless, from the viewpoint of today, Dalton contributed remarkably for his time, and his name continues to be revered by its association with the theory.