The History of the Atom

The History of the Atom

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons. Neutrons are neutrally charged, but protons and electrons are electrically charged: protons have a relative charge of +1 and electrons have a relative charge of -1.

Democritus (460-370 BCE)
The word atom comes from atomos, the Ancient Greek word for 'indivisible'. This is because the Greek philosopher Democritus maintained that all matter can be divided and sub-divided into smaller and smaller units, so that eventually there is a tiny particle that cannot be divided any further - an atom. This is considered remarkable because there is no way the Ancient Greeks could support this theory by observation or experiment.

John Dalton (1766 - 1844)
Understanding of atoms didn't progress much beyond Democritus' theory until Dalton's experiments in the early 1800s. He concluded that:

• 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.

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

JJ Thomson (1856 - 1940)
In 1897, Thomson concluded from his experiments that atoms aren't…