Question

why are the atomic masses of most elements are fractional​

Answer 1

The atomic masses of most elements are fractional because they exist as a mixture ofisotopes of different masses. ... Most elements occur as a mixture of isotopes of different masses. The fractional atomic masses arise because of this mixture.

Answer 2

That is because most of the elements have many isotopes. The mass of the element is the mass of its isotopes multiplied by its respective abundance fraction.

For example Chlorine has 2 isotopes one with mass 35 and one with 37their relative abundance is 75% and 25% respectively

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