1. The problem statement, all variables and given/known data The two isotopes of antimony have masses 120.90 and 122.90, with a relative abundance of 57.25% and 42.75% respectively. Using these figures, distinguish clearly between atomic mass, isotopic mass, and mass number. 2. Relevant equations 3. The attempt at a solution I think atomic mass is the relative isotopic abundance which in our case would be (0.5725)(120.90) + (.4275)(122.90) = 121.755 Would isotopic mass be simply 120.90 and 122.90? Would mass number be the same as the atomic mass? Some how I think I am wrong concerning the last two.