noun
: a number characteristic of an element and taken to represent the positive charge on the nucleus of an atom of the element, being equal to the number of protons in the nucleus and in a neutral atom to the number of electrons outside the nucleus, an atomic number as determined experimentally by X-ray spectra being assigned to each element and determining its place in the periodic table and its properties except those depending on atomic weight — symbol Z ; abbr. at. no. ; see element table : isotope , periodic table