The atomic masses of the atoms on the periodic table are not whole numbers because they are averages of all known isotopes of the element (variations of the same element with different numbers of neutrons). Since the number of neutrons affects the mass of the atom, the existence of different isotopes affects the listed atomic mass. Also, the averaged isotopic mass given on the periodic table is useful because from it, the relative prevalence of the different isotopes of an element can be determined. For example, the atomic mass of Boron is 10.8 amu, so we know that about 80% of Boron atoms have 6 neutrons, and about 20% have 5 neutrons