atomic num·ber -'nəm-bər n an experimentally determined number characteristic of a chemical element that represents the number of protons in the nucleus which in a neutral atom equals the number of electrons outside the nucleus and that determines the place of the element in the periodic table see ELEMENT (table)
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(Z) the number of protons in the nucleus of a nuclide; all the atoms of a chemical element have the same atomic number; sometimes indicated by a subscript preceding the symbol of a chemical element (e.g., 1H).Medical dictionary. 2011.