Final answer:
The atomic mass of an element is the weighted average of the masses of its isotopes, which have different mass numbers. The abundances of the isotopes are used to calculate the atomic mass.
Step-by-step explanation:
The atomic mass of an element is not the same as the mass number of one atom of the element because the atomic mass is the weighted average of the masses of the isotopes of an element. Isotopes of an element have different masses and are identified by their mass numbers. The atomic mass of an element is calculated using the relative abundances of the element's naturally occurring isotopes and the masses of those isotopes. If all the abundances are not provided, it is safe to assume that all numbers should add up to 100%.