Define the term “atomic mass” of an element

Atomic mass is the mass of a single atom, usually expressed in atomic mass units (u), and for elements it reflects the weighted average of the masses of all naturally occurring isotopes.

Atomic mass connects chemistry to physics. One atomic mass unit is defined based on carbon-12, where 1 u equals one-twelfth the mass of a carbon-12 atom. Elements exist as isotopes with different numbers of neutrons. Natural samples contain mixtures of isotopes, so the atomic mass on the periodic table is a weighted average. For example, chlorine has isotopes Cl-35 and Cl-37; its listed atomic mass is about 35.45 u.

Atomic mass helps calculate moles, molar mass, and stoichiometry in chemical equations. It also aids in spectroscopy and mass spectrometry analysis.