Atom, the smallest part of a chemical element that retains the identity of the element. The study of the structure and behavior of the atom by chemists and physicists has contributed greatly to the scientific understanding of matter and its properties. The use of this knowledge has greatly affected present-day technology, leading to the commercial development of electronics and nuclear energy and to the production of many new kinds of materials and products, including plastics and synthetic fibers.

Atoms are so small that a hundred million of them, placed side by side, would make a row only about one inch (2.5 cm) long. Individual atoms can be viewed only with certain types of nonoptical microscopes that have extremely high magnifications.

Each chemical element has its own chemical identity and behaves differently from every other element. Atoms of the same element are identical in chemical behavior; they differ from the atoms of other elements in chemical behavior, in structure, and (usually) in weight.