Since the times of the ancient Greeks, scientists and philosophers have generally agreed that everything that takes up space, or matter, is made up of tiny bits of material that have come to be known as elementary particles. The only thing that has changed over the years is the scale of what we mean by the term particle.

The Greek philosopher Democritus, who conceived of the particulate nature of matter, believed that what he called atoms (from the Greek word atomos for indivisible) were the smallest constituents of matter. However, scientific advances have shown us that there are at least several smaller kinds of particles that make up each atom. Nevertheless, the atom remains the most stable and convenient way to understand and discuss the nature of matter--especially since it is the smallest particle that can enter into chemical combination.

The atomic...