A unit of mass that describes the mass of an atom or molecule is the

• A. Atomic Number
• B. Mass Number
• C. Atomic Mass Unit
• D. Nuclear Force

Answer: (C)

Understanding how we measure tiny masses helps here. The unit used for the mass of atoms and molecules is the atomic mass unit (amu). It’s defined so that a carbon-12 atom has a mass of exactly 12 amu, which makes the scale convenient because many atoms weigh only a few amu and molecules are built by adding the masses of their constituent atoms. One amu is about 1.66 × 10^-27 kilograms, a scale appropriate for the world of atoms.

This unit is different from the numbers you might use to describe composition. The atomic number tells you how many protons are in an atom, not its mass. The mass number counts protons plus neutrons, but it isn’t a standardized mass unit by itself. The nuclear force is a type of interaction, not a unit of mass. Using amu lets you compare and sum the masses of atoms to get the overall mass of a molecule, and in practice the molar mass in grams per mole matches the mass in amu numerically, linking microscopic mass to macroscopic quantities.