Question

why are atomic masses generally fractional

Answer 1

The atomic masses of most elements are fractional because they exist as a mixture of isotopes of differentmasses. Most elements occur as a mixture of isotopes of differentmasses. The fractional atomic massesarise because of this mixture.

Answer 2

To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

Best example is

chlorine has an atomic weight of 35.5 because of something called isotopes.

Isotopes are atoms with the same number of protons (in the case of chlorine that means 17 protons) but different numbers of neutrons. Chlorine has two common isotopes, Chlorine-35 and Chlorine-37.