Question

why atomic mass number is a whole number

Answer 1

Atomic number is simply the count of protons in an atom’s nucleus, so it is always a whole number. Atomic mass is NOT a whole number, as it includesthe mass of protons, neutrons, and all the other sub-atomic particles in the atom. Generally atomic mass for a given element is stated as the average mass of the atoms of all the isotopes for that element weighted in proportion to relative abundance of the various isotopes as they occur in nature. For any particular isotope the atomic mass is very close to a whole number...

Answer 2

Atomic mass number is actually the addition of number of protons and neurons in an atom.

Now, because the number of protons and neutrons is always a whole number, so their sum will be also a whole number.

A = p + n

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Hope it was helpful.