Standard atomic weight

Standard atomic weight is the IUPAC-recommended relative atomic mass of an element in normal terrestrial materials. It accounts for natural variation in isotopic composition and may be reported as an interval.

Relative atomic mass: The ratio of an atom’s mass to one twelfth the mass of a carbon-12 atom. Standard atomic weight expresses an element’s representative atomic mass on this relative scale.
Weighted mean: An average in which each value contributes in proportion to an assigned weight. Isotope masses contribute according to their measured abundances in representative terrestrial materials.
Molar mass: The mass of one mole of a specified substance, expressed in mass per amount of substance. Standard atomic weights help calculate molar masses for elements and compounds.
Atomic mass: The mass of an individual atom, usually expressed in daltons or kilograms. An individual atom has a mass; standard atomic weight characterizes an element across materials.
John Dalton: An English chemist who developed an early modern atomic theory in the early nineteenth century. His atomic-weight tables helped establish relative atomic masses as a practical chemical framework.
Isotope: One of multiple forms of an element whose atoms have different numbers of neutrons. An element’s isotopes have different masses, creating variation in its standard atomic weight.
Isotope-ratio mass spectrometry: A mass-spectrometric method for measuring ratios between isotopes of an element. It measures isotope ratios used to establish atomic-weight values and their variability.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring chemical properties. Its element entries commonly display standard atomic weights alongside atomic numbers.
Mass number: The total number of protons and neutrons in a particular nuclide. Mass number is an integer for one nuclide, not an abundance-weighted element value.
Stanislao Cannizzaro: An Italian chemist who clarified the distinction between atomic and molecular weights in the nineteenth century. His work helped bring consistency to the atomic weights used by chemists.

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