Atomic weight

Atomic weight is the weighted average mass of an element’s atoms relative to a reference standard, usually expressed in atomic mass units. It reflects the abundances of the element’s naturally occurring isotopes.

Isotope: An atom of a given element with a particular number of neutrons in its nucleus. Each isotope contributes its own mass to the element’s weighted average.
Weighted mean: An average in which each value contributes according to an assigned weight. Atomic weight is calculated as a weighted mean of isotope masses.
Molar mass: The mass of one mole of a substance, usually expressed in grams per mole. Atomic weights provide the elemental values used to calculate molar masses.
Atomic mass: The mass of an individual atom, commonly expressed in daltons. An individual atom’s mass is not the abundance-weighted value called atomic weight.
Relative atomic mass: The ratio of an atom’s average mass to one-twelfth the mass of a carbon-12 atom. This dimensionless quantity is closely related to atomic weight and clarifies its reference scale.
Isotope-ratio mass spectrometry: A mass-spectrometric method for measuring ratios between isotopes of an element. It supplies precise isotope-abundance measurements used to determine atomic weights.
Stoichiometry: The quantitative relationships between reactants and products in chemical reactions. Accurate atomic weights convert chemical formulas into usable mass ratios.
Isotopic mass: The mass of a specific isotope, generally measured in daltons. Atomic weight averages isotopic masses rather than naming one isotope’s mass.
Carbon-12: A stable carbon isotope with six protons and six neutrons, defined to have an exact mass of 12 daltons. Its mass defines the modern atomic-mass reference standard.
Mass spectrometry: A method that identifies and measures ions according to their mass-to-charge ratios. It separates isotope signals that underlie many atomic-weight measurements.

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