Atomic mass is defined as what?

• A. The sum of protons and electrons in an atom.
• B. The total number of neutrons in the nucleus.
• C. The weighted average of the masses of the isotopes of an element.
• D. The number of protons in the nucleus.

Answer: (C)

Atomic mass is the average mass of an element’s atoms when you account for all naturally occurring isotopes and how abundant each one is. Isotopes are atoms with the same number of protons but different numbers of neutrons, so they have slightly different masses. The mass on the periodic table isn’t the mass of a single atom or just protons plus neutrons; it’s a weighted average of the isotopic masses, weighted by their natural abundances, measured in atomic mass units. For a single isotope, mass is roughly protons plus neutrons, but the actual atomic mass reflects the combined masses of all isotopes and their frequencies. For example, chlorine’s atomic mass comes from its two main isotopes and their proportions, giving about 35.45 u.