An Etymological Dictionary of Astronomy and Astrophysics

The number of → protons in an → atomic nucleus (symbol Z). Same as → Z-number.

The atomic number is written as a subscript to the left of the → chemical element name. For example, the most common isotope of oxygen is shown as ₈¹⁶O, which has 8 → protons and its → mass number (A) is 16.

See also: → atomic; → number.