Atomic number
In chemistry and physics, the
atomic number of a chemical element is the number of protons found in the nucleus of an atom of that element, and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol
Z. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons. The atomic number,
Z, should not be confused with the mass number,
A, which is the number of nucleons, the total number of protons and neutrons in the nucleus of an atom. The number of neutrons,
N, is known as the neutron number of the atom; thus,
A =
Z +
N. Since protons and neutrons have approximately the same mass and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units is roughly equal to the whole number
A. Atoms with the same atomic number
Z but different neutron numbers
N, and hence different atomic masses, are known as isotopes.