Mass Number: Why Can It Be a Fraction?

[kooyewyoung]

Homework Statement

My instructor give me a question:
Mass number (number of protons and number of neutrons in an atom) is supposed to be a whole number for a particular atom, in reality IT IS NOT. Why is it so?

Homework Equations

The Attempt at a Solution

Here, I am very sure that this question ask for mass number (nucleon number) not atomic weight. However, how can only a part of a proton or neutron exist in an atom if mass number are not a whole number?

 

[SteamKing]

Hint: Check out isotopes.

 

[technician]

you are absolutely correct. Mass number is the NUMBER of neutrons and protons (nucleons)
This does not mean that the MASS is a whole number.
Like steamking I suggest you look at isotopes to get the idea of mass