Question

Many elements have fractional atom mass why?

Answer 1

Answer:

If you mean why aren't they whole numbers? It has to do with relative abundances of isotopes.

For example, on Earth, Chlorine exists in two isotopes. Approximately 76% of all chlorine atoms are Cl-35, and 24% are Cl-37. The atomic masses given in the periodic table are designed to be useful in stoichiometric calculations- they tell you exactly what a mole of atoms of a certain element, in typical earth composition, will weigh.

Thus, a typical mole of chlorine atoms will consist of 76% Cl-35 atoms, 24% Cl-37 atoms, and will weigh 35.45 grams.