Final answer:
The atomic mass of elements on the Periodic Table is a weighted average of all stable isotopes' masses and their relative abundances. For radioactive elements without stable isotopes, the mass number of the most stable isotope is given instead.
Step-by-step explanation:
The atomic mass of most elements on the Periodic Table is not an integer because these elements exist as mixtures of several stable isotopes. The atomic mass listed is the weighted average of the masses of these isotopes. This average is calculated by taking into consideration the mass and the relative abundance of each isotope present in nature. For example, chlorine (Cl) has a atomic mass of 35.45, reflecting the mix of isotopes Cl-35 and Cl-37 in natural chlorine. This calculated mean represents a weighted average, which is generally closest to the mass number of the most abundant isotope.
However, for elements like technetium (element 43), promethium (element 61), and those with atomic numbers 84 and above, which consist entirely of unstable, radioactive isotopes, an average atomic weight cannot be determined. In such cases, the atomic mass given in square brackets in the periodic table reflects the atomic mass number of the most stable isotope for that element.