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Atomic theory states that matter is made up of small units called atoms, which were originally thought to be indivisible. However, British scientist JJ Thomson discovered that atoms are made up of smaller particles. John Dalton’s atomic theory had five main points, most of which are still accepted today, but with some exceptions due to the discovery of nuclear fusion/fission and isotopes. Dalton’s theory was popularized and confirmed experimentally during the early 19th century.
Atomic theory is the idea that matter is made up of small units called atoms. When the ancient Greek philosopher Democritus came up with the idea in the 5th century BC, it was originally intended to refer to indivisible units. As of 5, British scientist JJ Thomson discovered that atoms are actually made up of smaller particles. Today this theory refers to the fact that matter is composed of units that are indivisible only a few times. Exceptions include plasmas, such as fire, other ionic arrangements, such as those found in the body, radioactive materials, and many others.
Even though today atomic theory is a familiar cornerstone of modern science, like germ theory or evolution, for much of human history, people believed that matter was probably continuous and could be broken down into arbitrarily small quantities. It was not until 1803, or perhaps a little earlier, that the English chemist John Dalton took up the old idea and used it to solve various problems chemists were grappling with at the time. Rather than one experiment leading up to the idea, it emerged from the analysis of multiple experiments involving gas properties and chemical reactions. His theory was popularized and confirmed experimentally during the early 19th century.
Dalton’s atomic theory had five main points:
All matter is made up of tiny particles called atoms.
All atoms of a given element are identical to each other.
All atoms of a given element are different from those of other elements.
Atoms of an element combine with other elements to make compounds. They always combine in equal amounts.
Atoms cannot be created, divided or destroyed.
Most of the above is still accepted by scientists today, with the exception of a few points. First, the discovery of nuclear fusion/fission and radioactivity prompted the revision of point 2. Isotopes demonstrate that atoms of the same element can actually have small differences due to different numbers of neutrons. Furthermore, the existence of ions with varying numbers of electrons also contradicts this point.
The fifth point is also invalidated by nuclear physics, as atoms can in fact be destroyed in nuclear chain reactions. The second point of point #4 is also completely wrong, since, for example, water is H2O, not HO. His insistence that atoms combine in equal amounts to create compounds prevented acceptance of his theory for years. Regardless, from today’s perspective, Dalton contributed greatly to his time, and his name continues to be revered for its association with the theory.
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