text: The point is that the mass shown on the periodic table for each element is in fact an average that takes into account the most abundant isotope (the one actually shown in the table) along with other less abundant isotopes (typically radioactive ones) that most often differ in one neutron or two. \n\nFor example, Hidrogen´s atomic mass should be just 1. But it is slightly higher than that. That´s because H1 is only the major form of Hydrogen. There are also H2 (deuterium) and H3 (Tritium). And the atomic mass averages the mass of all of these (1,2 and 3 respectively) according to their abundances. \nHere are their abundances:\n\nH1 (Common Hydrogen)= 99.985%\nH2 (Deuterium)= 0.015%\nH3 (Tritium)= minimal traces\n\nThus for a weighed average you need to calculate:\n\n1*0.99985+2*0.00015+3*0= 1.00015 which is a bit higher than just 1.\n\nThe same goes for the rest of the elements. \n\nExcelent question!