Atomic Mass Unit Definition (AMU)

In chemistry, an atomic mass unit or AMU is a physical constant equal to one-twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses. When the mass is expressed in AMU, it roughly reflects the sum of the number of protons and neutrons in the atomic nucleus (electrons have so much less mass that they are assumed to have a negligible effect). The symbol for the unit is u (unified atomic mass unit) or Da (Dalton), although AMU may still be used.

1 u = 1 Da = 1 amu (in modern usage) = 1 g/mol

Also Known As: unified atomic mass unit (u), Dalton (Da), universal mass unit, either amu or AMU is an acceptable acronym for atomic mass unit

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Atomic Mass Definition: Atomic Weight

By Anne Marie Helmenstine, Ph.D.

The "unified atomic mass unit" is a physical constant that is accepted for use in the SI measurement system. It replaces the "atomic mass unit" (without the unified part) and is the mass of one nucleon (either a proton or a neutron) of a neutral carbon-12 atom in its ground state. Technically, the amu is the unit that was based on oxygen-16 until 1961, when it was redefined based on carbon-12. Today, people use the phrase "atomic mass unit," but what they mean is "unified atomic mass unit."

One unified atomic mass unit is equal to:

• 1.66 yoctograms
• 1.66053904020 x 10^-27 kg
• 1.66053904020 x 10^-24 g
• 931.49409511 MeV/c²
• 1822.8839 m_e

History of the Atomic Mass Unit

John Dalton first suggested a means of expressing relative atomic mass in 1803. He proposed the use of hydrogen-1 (protium). Wilhelm Ostwald suggested that relative atomic mass would be better if expressed in terms of 1/16th the mass of oxygen. When the existence of isotopes was discovered in 1912 and isotopic oxygen in 1929, the definition based on oxygen became confusing. Some scientists used an AMU based on the natural abundance of oxygen, while others used an AMU based on the oxygen-16 isotope. So, in 1961 the decision was made to use carbon-12 as the basis for the unit (to avoid any confusion with an oxygen-defined unit). The new unit was given the symbol u to replace amu, plus some scientists called the new unit a Dalton. However, u and Da were not universally adopted. Many scientists kept using the amu, just recognizing it was now based on carbon rather than oxygen. At present, values expressed in u, AMU, amu, and Da all describe the exact same measure.

Examples of Values Expressed in Atomic Mass Units

• A hydrogen-1 atom has a mass of 1.007 u (or Da or amu).
• A carbon-12 atom is defined as having a mass of 12 u.
• The largest known protein, titin, has a mass of 3 x 10⁶ Da.
• AMU is used to differentiate between isotopes. An atom of U-235, for example, has a lower AMU than one of U-238, since they differ by the number of neutrons in the atom.