A unit of mass used to express atomic and molecular weights, equal to one twelfth of the mass of an atom of carbon-12.
- 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).
- 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.
- 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.
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