atom, the smallest unit of a chemical element having the properties of that element. An atom contains several kinds of particles. Its central core, the nucleus, consists of positively charged particles, called protons, and uncharged particles, called neutrons held together by the strong force. Surrounding the nucleus and orbiting it are negatively charged particles, called electrons. Each atom has an equal number of protons and electrons unless it has been ionized. The nucleus occupies only a tiny fraction of an atom's volume but contains almost all of its mass. Electrons in the outermost orbits determine the atom's chemical and electrical properties. The number of protons in an atom's nucleus is called the atomic number and determines which element it is. The number of nucleons (protons and neutrons in the nucleus) is the atom's mass number. Atoms containing the same number of protons but different numbers of neutrons are isotopes of the element. The atomic weight is the average mass of an atom of an element in atomic mass units. One atomic mass unit (1 AMU=1.6606x10^-27kg) is 1/12th the mass of the C¹² isotope. The atomic weight may involve an average over several naturally occuring isotopes of the element. The mass of atoms other than hydrogen is different from the sum of the masses of the neutrons and protons in the nucleus because of the binding energy of the nucleus. See also history of the atom, standard units.