Elements, atoms, molecules and substances

‘Like all other scientific concepts, that of an element has changed its meaning many times and in many ways during the development of science.’ Thus wrote Wilhelm Ostwald (the father of physical chemistry and a positivist philosopher) in the 1911 edition of the Encyclopaedia Britannica. This was a time of dramatic developments in physics and chemistry; within a few years, even the most entrenched positivists were beginning to believe in the real existence of atoms and subatomic particles.

Unfortunately, both classical and modern meanings of the term ‘element’ are still found in the literature, without proper discrimination. Most contemporary dictionaries define an element as a substance — a material having measurable bulk properties such as density, melting/boiling points, electrical conductivity, specific heat etc. — which cannot be further divided by chemical means. This stems from the classification by Lavoisier and Dalton (c. 1800) of substances into elements and compounds. But in almost any modern text, you will find usage of the term ‘element’ which may seem confusing and contradictory. The word has retained its classical meaning to imply a substance, but has assumed a much broader significance.

You may, for example, see it written that ‘aluminium is one of the most abundant elements in the Earth's crust’.