What is the primary difference between atomic mass and atomic number?

• A. Atomic mass and atomic number are the same.
• B. Atomic mass is always a whole number, while atomic number is not.
• C. Atomic mass is the average mass of all isotopes, while atomic number is the number of protons.
• D. Atomic mass is the number of protons, while atomic number is the average mass of all isotopes.

Answer: (C)