the mass number of chlorine is quoted on the periodic table as 35.5 why is this not a whole number

For example, the relative atomic mass of chlorine is 35.5 rather than a whole number. This is because chlorine contains two different isotopes, chlorine-35 and chlorine-37. The table shows the mass numbers and abundances of the isotopes of copper in a naturally-occurring sample.

Explanation:

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The reason for the atomic mass of chlorine taken as 35.5u and not as 35u because of the presence of isotopes. Isotopes are defined as variants of a particular element where these variants will have the same number of protons but differ in the number of neutrons in the atom.

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