Final answer:
Atomic mass is the weighted average mass of an element's isotopes in atomic mass units (amu) or grams per mole, while atomic weight is an older term often referencing the same concept but used to emphasize a standard weight comparison against carbon-12.
Step-by-step explanation:
The difference between atomic mass and atomic weight is a frequent source of confusion in chemistry. Atomic mass is essentially the weighted average of the masses of the isotopes present in a natural sample of the element. It is expressed in atomic mass units (amu), which is defined as one twelfth the mass of a carbon-12 atom. On the periodic table, atomic mass is also the mass of one mole of the element in grams, hence the term molar mass for this concept as well. Atomic weight, on the other hand, is a somewhat outdated term that refers to the same concept as atomic mass, but it is often used to emphasize the standard weight comparison of elements based on the natural isotopic distribution of the element as found on Earth. It is a dimensionless quantity since it's a ratio compared to the defined standard of carbon-12.
For example, an atom of carbon-12 is assigned an atomic mass of exactly 12amu. A single oxygen-16 atom has an atomic mass of 16amu, and the periodic table would then report the atomic mass of oxygen based on the weighted average of its naturally occurring isotopes, which is approximately 15.999amu, close to the mass number of its most stable isotope, oxygen-16.