atomic mass

American  

[uh-tom-ik mas] / əˈtɒm ɪk ˈmæs /

noun

Chemistry.

1. the mass of an isotope of an element measured in units formerly based on the mass of one hydrogen atom taken as a unit or on 1/16 (0.0625) the mass of one oxygen atom, but after 1961 based on 1/12 (0.0833) the mass of the carbon-12 atom.

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atomic mass British  

noun

1. the mass of an isotope of an element in atomic mass units

2. short for relative atomic mass See atomic weight

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atomic mass Scientific  

1. The mass of a given atom or molecule, expressed in atomic mass units.

2. Compare atomic weight See also mass number

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Etymology

Origin of atomic mass

First recorded in 1895–1900

Compare meaning

How does atomic-mass compare to similar and commonly confused words? Explore the most common comparisons:

• atomic weight vs. atomic mass
• atomic mass vs. atomic number
• mass number vs. atomic mass

Example Sentences

Potassium occurs naturally in three isotopic forms -- potassium-39, potassium-40, and potassium-41 -- each differing slightly in atomic mass.

From Science Daily • Oct. 17, 2025

Wallner is particularly interested in the radioactive iron isotope with the atomic mass of 60.

From Science Daily • Nov. 2, 2023

Twelve, named for the atomic mass of carbon, has developed a process to make jet fuel using renewable electricity, water and waste biomass CO2 instead of fossil fuels.

From Seattle Times • Jul. 12, 2023

The protein and synthetic polymer molecules that form colloids may have molecular masses ranging from a few thousand to many million atomic mass units.

From Textbooks • Feb. 14, 2019

The best evidence that these particles are of atomic mass is furnished by their deflection in electric and magnetic fields.

From A Brief Account of Radio-activity by Venable, Francis Preston