Molecule

Lore & Background

Concepts similar to molecules have been discussed since ancient times, with Greek philosophers such as Leucippus and Democritus arguing that the universe is composed of atoms and voids. These ideas, along with the concept of aether, were accepted by Aristotle and passed to medieval and Renaissance Europe. In 1927, Fritz London and Walter Heitler applied quantum mechanics to the hydrogen molecule, bringing chemistry under quantum mechanics.

Reader's Guide

The concept of the molecule has evolved from ancient philosophical speculation to a central pillar of modern science. The definition has become more precise as knowledge of molecular structure increased, though earlier definitions—defining molecules as the smallest particles of pure chemical substances that retain their composition and properties—often break down for substances like rocks, salts, and metals, which are composed of large crystalline networks rather than discrete molecules. The modern understanding, refined by scientists such as Robert Boyle, Amedeo Avogadro, Jean Perrin, and Linus Pauling, underpins molecular physics and molecular chemistry. Molecules are common components of matter, making up most of the oceans and atmosphere, as well as most organic substances and the substances of life, such as proteins, nucleic acids, sugars, and fats. However, most familiar solid substances on Earth—minerals, sand, clay, rocks, metals, and glasses—are not made of identifiable molecules but of crystals, ionic compounds, or disordered networks. The distinction between molecules and ions is sometimes dropped in quantum physics, organic chemistry, and biochemistry, where polyatomic ions are often referred to as molecules.

Did You Know?

• In 1927, physicists Fritz London and Walter Heitler applied quantum mechanics to the hydrogen molecule, a landmark that brought chemistry under quantum mechanics.