- Atomic mass is determined by the number of protons and neutrons
- Atomic number is only determined by the number of proton of an element
- Hence, the difference between atomic mass and atomic number is the number of the neutrons in an element

So, the difference between atomic mass and atomic number is the number of the neutrons in an element
Question: The difference between the atomic number of an element and the element's atomic mass is the number of
Context: - Atomic mass is determined by the number of protons and neutrons 
- Atomic number is only determined by the number of proton of an element