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