The atomic masses of the elements are not whole number. atomic mass is roughly equal to the sum of the individual particle masses of an atom. the percent nature abundance of the elements ,that means that it is a weighted  average of all naturally occuring isotopes of that elements..

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Atomic mass of an element is defined as the sum of the number of protons and neutrons in the nucleus of its atom. Example: atomic mass of calcium is 40(20+20).
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