Atomic mass

Atomic mass is the mass of an individual atom, usually expressed in unified atomic mass units. It depends on the atom’s isotopes and, for a particular isotope, includes the masses of its protons, neutrons, and electrons.

Isotope: An isotope is a form of an element whose atoms have the same proton count but different neutron counts. Different isotope proportions give samples of an element different average atomic masses.
Mass spectrometry: Mass spectrometry measures ions’ mass-to-charge ratios by separating them according to their motion in electric or magnetic fields. It resolves isotope masses and their relative abundances.
Molar mass: Molar mass is the mass of one mole of a substance, usually expressed in grams per mole. Atomic masses provide the values summed to calculate a compound’s molar mass.
Mass number: Mass number is the integer count of protons and neutrons in a particular nuclide. It is a count, not the measured mass of an atom.
John Dalton: John Dalton was an English chemist and physicist who developed an early scientific atomic theory in the early nineteenth century. His relative atomic weights made atomic mass central to chemical formulas.
Unified atomic mass unit: The unified atomic mass unit is defined as one twelfth of the mass of a neutral carbon-12 atom in its ground state. Atomic masses are commonly reported in this unit.
Isotope-ratio mass spectrometry: Isotope-ratio mass spectrometry measures the relative abundances of isotopes in a sample. Those abundance measurements determine an element’s weighted average atomic mass.
Stoichiometry: Stoichiometry is the quantitative study of relationships among reactants and products in chemical reactions. Atomic masses convert measured substance masses into amounts of atoms and molecules.
Monoisotopic mass: Monoisotopic mass is the mass of a molecule calculated using the most abundant isotope of each element. It describes a selected isotopic composition rather than an element-wide average.
Carbon-12: Carbon-12 is the stable carbon isotope with six protons and six neutrons, defined to have a mass of exactly 12 daltons. Its mass established the modern atomic-mass reference.

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