The atomic mass of an element is the weighted average of the atomic masses of different isotopes of the element. This explains, why . . .

Question : The atomic mass of an element is the weighted average of the atomic masses of different isotopes of the element. This explains, why atomic masses of many elements show large departures from integer values. However, even if we consider masses of individual isotopes, they are not strictly integral multiples of the mass of a hydrogen atom. How do you account for this fact?


Doubt by Aryan

Solution : 
Because of the following reasons 
masses of individual isotopes, are not strictly integral multiples of the mass of a hydrogen atom (proton)

a) The mass of proton and neutrons are not exactly same. 

b) The atomic mass also includes mass of electrons and an atom of mass A has Z electrons and not A electrons.
c) The mass of nucleus is slightly less than the mass of the constituent nucleons.