Which term is defined as the weighted average of the atomic masses of an element's isotopes?

• A. average atomic mass
• B. mole
• C. Avogadro's number
• D. molar mass

Answer: (A)

The idea being tested is that the average atomic mass represents the weighted average of an element’s isotopic masses. Each element exists as a mix of isotopes, each with its own mass and natural abundance. To get the average atomic mass, you multiply each isotope’s mass by its fractional abundance and sum these values. The result is the value you see on the periodic table for the element, expressed in atomic mass units. This concept explains why the number isn’t a single isotope’s mass, but a weighted blend reflecting what occurs in nature. For example, chlorine has two common isotopes in certain abundances, and the weighted average of their masses gives about 35.45 amu. It’s distinct from molar mass (grams per mole), which applies to any substance, and from Avogadro’s number or the mole concept, which count particles rather than average masses.