Believe it or not, there are actually several atomic mass units … however, the one that’s standard – throughout chemistry, physics, biology, etc – is the unified atomic mass unit (symbol u). It is defined as 1/12 (one-twelfth) of the mass of an isolated carbon-12 atom, in its ground state, at rest. You’ll still sometime see the symbol amu – which stands for atomic mass unit – but that’s actually two, slightly different, units (and each is different from the unified atomic mass unit!) … these older units are defined in terms of oxygen (1/16th of an isolated oxygen-16 atom, and 1/16th of an ‘average’ oxygen atom).
As it’s a unit of mass, the atomic mass unit (u) should also have a value, in kilograms, right? It does … 1.660 538 782(83) x 10-27 kg. How was this conversion worked out? After all, the kilogram is defined in terms of a bar of platinum-iridium alloy, sitting in a vault in Paris! First, it is important to recognize that the unified atomic mass unit is not an SI unit, but one that is accepted for use with the SI. Second, the kilogram and unified atomic mass unit are related via a primary SI unit, the mole, which is defined as “the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12“. Do you remember how many atoms there are in a mole of an element? Avogadro’s number! So, work out the Avogadro constant, and the conversion factor follows by a simple calculation …
The Dalton (symbol D, or Da) is the same as the unified atomic mass unit … why have two units then?!? In microbiology and biochemistry, many molecules have hundreds, or thousands, of constituent atoms, so it’s convenient to state their masses in terms of ‘thousands of unified atomic mass units’. That’s far too big a mouthful, so convention is to use kDa (kilodaltons).
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Find out more on the (unified) atomic mass unit, from the Argonne National Laboratory, from the International Union of Pure and Applied Chemistry, and from the National Institute of Standards and Technology (NIST).
Learning to Breathe Mars Air and Mini-Detector Could Find Life on Mars or Anthrax at the Airport are two Universe Today articles relevant to the atomic mass unit.
Energy Levels and Spectra and Inside the Atom are two Astronomy Cast episodes related to the atomic mass unit.
Newton Ask a Scientist