The subatomic particles which comprise an atom were reviewed before moving on to atomic number and atomic mass. The atomic number was defined as the number of protons in the nucleus, and each proton was given a mass unit weight of 1 atomic mass unit. It was explained that this is not the actual mass, just a method to compare the masses. It was pointed out from the periodic table that each element contained a specific, individual number of protons, which was accorded its atomic number. If an atom contained a different number of protons, it was not the atom being discussed. The atomic mass was defined as the number of protons and neutrons in the nucleus of the atom. A neutron was afforded an atomic mass unit weight on 1 AMU, the same as a proton. That atoms of different elements may have the same number of protons and that this did not affect what it was, only the number of protons determine what an element is. The term isotope was introduced for discussion at the next class.