Question

4.) why is mass number a whole number, but atomic mass a decimal number?

Explanation:

The mass number is the sum of protons and neutrons in an atom, which are counted as whole - entities. Atomic mass is the weighted average of the masses of an element's isotopes, considering their relative abundances, which can result in a decimal number.

Answer:

The mass number is a count of protons and neutrons (whole entities), while atomic mass is a weighted - average of isotope masses, leading to a decimal value.