In the fifth century B.C., Empedocles, a Greek philosopher, developed the theory that all matter in the universe was composed of various combinations and proportions of four elementary substances—earth, air, fire, and water. Metals, for example, were considered to be composed of earth and fire, since they could be produced by placing ores (earth) in a flame. The shinier a metal, the more fire it was believed to contain.

The Greek philosopher Democritus (460?-370 B.C.) accepted the four-element theory of Empedocles and taught that all matter was composed of tiny, indestructible, solid particles (which he called atoms) of earth, air, fire, and water. Democritus believed that spiritual, as well as physical, things were composed of atoms and that atoms could not be subdivided. ("Atom" comes from the Greek word atomos, which means "indivisible.") Democritus was not the first atomist (as persons who subscribed to his views are called), but he had great influence.

The atomism of the early Greeks, although mentioned by such men as Giordano Bruno (1548-1600), Sir Isaac Newton (1642-1727), and Daniel Bernoulli (1700-1782), was largely neglected for more than 2,000 years. John Dalton of England originated the modern atomic theory in the early 19th century. Dalton's description of the atom as a submicroscopic, solid, nondivisible particle of a chemical element seemed to explain satisfactorily the properties of matter that were known at that time.

The concept of solid atoms, although now outmoded, is still useful in diagramming certain chemical changes and in explaining the nature of molecules. For example, ball-like models of various kinds of atoms are joined together to show the structure of large molecules.

In 1897 the English physicist J. J. Thomson determined the nature of electrons—tiny particles with negative electric charges—and measured their mass. He proposed that an atom was made up of a number of electrons embedded in a sphere of positive electricity.

In 1911 the British physicist Ernest Rutherford proposed the nuclear atom. According to his theory, an atom consists of a small, dense, positively charged nucleus (center) with electrons revolving around it. The atom is mostly empty space, because the diameter of the nucleus is much smaller than the total diameter of the atom. In 1913, the Danish physicist Niels Bohr expanded upon this theory. According to Bohr, the electrons circle the nucleus in fixed orbits, and when an atom gains or loses energy the electrons jump from one orbit to another.

Present-day atomic theory is largely based on concepts proposed in the 1920's by the Austrian Erwin Schrdinger, the Germans Werner Heisenberg and Max Born, Paul Dirac of Great Britain, and Louis de Broglie of France. These physicists were pioneers in the field of quantum mechanics.

The atom of quantum mechanics is difficult to picture and is best described mathematically. The negative charge of electrons is treated as a type of wave, to yield equations whose solutions give the probability of finding an electron in various regions around the nucleus. An electron's position and velocity cannot both be known accurately at the same time. The electron is sometimes pictured as a negatively charged cloud covering a large area outside the nucleus; in the region where the probability of finding the electron is greatest, the cloud is densest (and the concentration of charge is the highest).