


Atoms and atomic structure

Atoms consist of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] surrounding a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] that contains protonsprotons: Sub-atomic particles with a positive charge and a relative mass of 1. and neutronsneutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton..

Protons and neutrons have a relative massrelative mass: The relative mass is the number of times heavier a particle is, compared to another. of 1 and electrons have a negligiblenegligible: So small as to be not worth considering. mass. Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1 and electrons have a relative charge of -1.

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 moleculesmolecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds. His most important conclusions are summarised below.

  • atoms are rearranged in a chemical reaction
the atoms are represented as spheres and joined by a double bond

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


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