Question

give scientific reason of 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 includes the 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.