Factors contributing to the atomic mass of an atom

[Avalon_18]

I was studying about atomic masses and realized that even if we say that the atomic mass unit corresponds to 1/12 of the mass of a carbon atom. why is it that even particular isotopes of elements have atomic mass in decimal values. 1/12 of a carbon atoms mass should equal to the mass of a neutorn or a proton (the mass of an hydrogen atom). It isn't that I am confused with the concepts of atomic mass and mass number. my question is what is it that is contributing to the mass of an atom such that the atomic mass is in decimal values?

 

[Bandersnatch]

Binding energy.

 

[anorlunda]

Binding energy.

That's correct, but a B level answer was required.

@Avalon_18 , Think about nuclear energy. When we split heavy nuclei (fission) we release energy. When we fuse light nuclei (fusion) we release energy. When energy is released those way, the mass of the remaining particles is less than the starting mass. Mass is converted to energy. I'm sure you heard of E=mc².

The process can work in reverse, where we use energy to create nuclei with more mass than they started with.

The technical word is what @Bandersnatch said, "binding energy." You can study it more at
https://en.wikipedia.org/wiki/Binding_energy

 

[Anand Sivaram]

https://en.wikipedia.org/wiki/Standard_atomic_weight
There is a factor of isotope abundance also. The Wikipedia link above shows Copper has an atomic weight of 63.55 based on Cu-63 is 69% and Cu-65 is 31% abundance.