Nuclei contain uncharged neutrons and positively charged protons; these two particles with about the same mass are known as nucleons. The number of protons is the atomic number of an element (Z), and is matched in a neutral atom by the exact number of electrons. The total number of nucleons is the mass number and is specified by a superscript on the symbol of the element. Thus 1H has a nucleus with no neutrons and one proton, 16O has eight protons and eight neutrons, 208Pb has 82 protons and 126 neutrons.