Atomic theory and the concept of atoms was well known among the ancient Greeks. The most highly developed theory was that of Democritus of Abdera. Democritus was probably born about 460 b.c. and died about 370 b.c. He is reputed to have written more than 70 books, although almost none of his writing survives. Most of what we know of Democritus comes from the writings of those who came after him.

Democritus argued that all matter consists of tiny, physically invisible particles. The Greek word atomos, in fact, means "indivisible." Democritus taught that an infinite number of atoms exist and that they are in constant motion. The space between atoms, he said, is occupied by a void. Atoms were never created, according to Democritus, but have always existed, just as they are now. They are also eternal; that is, they cannot be destroyed.. Atoms have physical properties that explain...