This picture illustrates the concepts "atomic mass" and "atomic number". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons, giving it an atomic number of 6 (the number of protons) and an atomic mass (protons plus neutrons) of 14. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus, giving it an atomic number of 1 and an atomic mass of 3.
Original artwork by Windows to the Universe staff (Randy Russell).