The Atomic Mass Unit
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The atomic mass unit, or dalton, is defined as exactly one-twelfth the mass of a single carbon-12 atom.
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A proton's mass, about 1.0073 u, and a neutron's, about 1.0087 u, are both measured against this one shared carbon-12 reference.
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"Amu" is the older term for the same unit IUPAC now standardizes as the dalton (Da); both names describe one identical definition.