Early ideas about atoms

The word atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] comes from atomos, an ancient Greek word meaning indivisible. The Greek philosopher Demokritos (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom. This was remarkable because there was no way ancient Greeks could support this theory by observation or experiment.

John Dalton

John Dalton (1766-1844)

Understanding of atoms didn’t progress much beyond Demokritos’ theory until the English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ]. His most important conclusions are summarised below.

• all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces
• all the atoms of a particular element are identical to each other and different from the atoms of other elements
• atoms are rearranged in a chemical reaction
• compounds are formed when two or more different kinds of atoms join together

Molecules of oxygen - the two atoms are joined by a double bond

A molecule of carbon dioxide. Atoms are represented as spheres and are colour-coded - carbon (green) and oxygen (red). The atoms are joined by double bonds

Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.