How is the average atomic mass calculated from isotopes? Choose the correct method.

• A. Sum of masses divided by number of isotopes.
• B. Average of the most abundant isotope’s mass.
• C. Multiply total mass by abundance.
• D. Sum of products of isotopic abundances and isotopic masses.

Answer: (D)

The average atomic mass is a weighted average of the masses of all naturally occurring isotopes, with each mass weighted by how abundant that isotope is. To get it, multiply each isotope’s mass by its fractional abundance, then add all those products together. In other words, sum the products of isotopic abundances and isotopic masses. Abundances are the fractions (or percentages) that reflect how common each isotope is, and they sum to 1 (or 100%). This weighting is why the average isn’t just the mass of the most abundant isotope or a simple arithmetic mean of the masses. For example, chlorine’s two main isotopes contribute to the average mass according to their abundances, yielding about 35.45 amu. Using a simple average or multiplying a total mass by a single abundance would ignore the contribution of the other isotopes.