Charge number

Charge number (z) refers to a quantized value of electric charge, with the quantum of electric charge being the elementary charge, so that the charge number equals the electric charge (q) in coulombs divided by the elementary-charge constant (e), or z = q/e. The charge numbers for ions (and also subatomic particles) are written in superscript, e.g. Na⁺ is a sodium ion with charge number positive one (an electric charge of one elementary charge). Atomic numbers (Z) are a special case of charge numbers, referring to the charge number of an atomic nucleus, as opposed to the net charge of an atom or ion. All particles of ordinary matter have integer-value charge numbers, with the exception of quarks, which cannot exist in isolation under ordinary circumstances (the strong force keeps them bound into hadrons of integer charge numbers).