Definition of atomic mass unit in US English:

atomic mass unit

(also amu)


  • A unit of mass used to express atomic and molecular weights, equal to one-twelfth of the mass of an atom of carbon-12. It is equal to approximately 1.66 x 10⁻²⁷ kg.

    • ‘Protons and neutrons have masses of approximately 1 atomic mass unit each, whereas electrons only have a mass of .0006 amu.’
    • ‘A Dalton, also known as an atomic mass unit, is roughly the mass of a single proton or neutron.’
    • ‘The electron is much smaller than either nuclear particle or ‘nucleon’, with a mass of only 0.0005 amu.’
    • ‘An electron's mass is so small that it is usually given a value in atomic mass units, compared to the value of 1 assigned to neutrons and protons.’
    • ‘The proton is a positively charged particle, while the neutron is neutral, but they have similar masses, the neutron being heavier than the proton by only one-thousandth of an atomic mass unit.’
    • ‘First, neutrons and protons don't happen to weigh exactly one amu apiece; the proton actually weighs 1.0072765 amu and the neutron weighs 1.0086650 amu.’
    • ‘At the other end, an atomic mass unit equals 1.66 yoctograms.’
    • ‘This standard was adopted internationally in 1961, replacing an arbitrarily assigned value of 16.000 amu for the atomic mass of an atom of oxygen.’
    • ‘The mass standard in the atomic world is set by one atom of the element carbon, which is defined to be 12 atomic mass units.’
    • ‘For example, carbon 12 has a mass of exactly 12 amu, but it is made up of six protons and 6 neutrons (1.0087 amu each).’