Relative atomic mass

Relative atomic mass is the weighted mean mass of an element’s atoms relative to one-twelfth the mass of a carbon-12 atom. It is dimensionless and reflects the abundances of the element’s isotopes.

Isotope: An isotope is a form of an element whose atoms have the same number of protons but different numbers of neutrons. Each isotope contributes a distinct mass to the element’s weighted mean.
Weighted arithmetic mean: A weighted arithmetic mean is an average in which each value contributes in proportion to its assigned weight. Relative atomic mass is calculated by weighting isotope masses by their abundances.
Molar mass: Molar mass is the mass of one mole of a substance, usually expressed in grams per mole. An element’s relative atomic mass gives its molar mass numerically in grams per mole to usual precision.
Mass number: Mass number is the integer count of protons and neutrons in the nucleus of a particular nuclide. Unlike relative atomic mass, it describes one nuclide and is not an isotope-weighted mean.
Relative isotopic mass: Relative isotopic mass is the mass of an atom of a specific isotope relative to one-twelfth the mass of a carbon-12 atom. It supplies the isotope-specific masses averaged to obtain relative atomic mass.
Mass spectrometry: Mass spectrometry measures ions according to their mass-to-charge ratios and can reveal isotope masses and abundances. It provides the measurements used to determine an element’s isotope contributions.
Standard atomic weight: Standard atomic weight is the recommended relative atomic mass for an element in normal terrestrial materials, often given as an interval. It extends the isotope-weighted idea to natural samples whose isotope proportions can vary.
Atomic mass: Atomic mass is the mass of an individual atom, commonly expressed in unified atomic mass units or kilograms. Relative atomic mass is a dimensionless ratio, whereas atomic mass carries units.
Isotopic abundance: Isotopic abundance is the proportion of atoms of an element that belong to a particular isotope. These proportions determine how strongly each isotope’s mass affects the mean.
Mass spectrum: A mass spectrum plots ion signal against mass-to-charge ratio, often displaying distinct isotope peaks. Peak positions and intensities can yield isotope masses and relative abundances.

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