The
unified atomic mass unit (Also known as
amu) (symbol:
u) or
dalton (symbol:
Da) is a unit that is used for indicating
massMass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of
carbon12Carbon12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
in its nuclear and electronic
ground stateThe ground state of a quantum mechanical system is its lowestenergy state; the energy of the ground state is known as the zeropoint energy of the system. An excited state is any state with energy greater than the ground state...
, and has a value of . One dalton is
approximately equal to the mass of one proton or one neutron. The CIPM have categorised it as a "non
SIThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit whose values in SI units must be obtained experimentally".
History
The
atomic weightAtomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon12...
scale has traditionally been a relative scale, that is without an explicit unit, with the first atomic weight basis suggested by
John DaltonJohn Dalton FRS was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness .Early life:John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland,...
in 1803 as
^{1}H. Despite the initial mass of
^{1}H being used as the natural unit for atomic weight, it was suggested by
Wilhelm OstwaldFriedrich Wilhelm Ostwald was a Baltic German chemist. He received the Nobel Prize in Chemistry in 1909 for his work on catalysis, chemical equilibria and reaction velocities...
that atomic weights would be best expressed in terms in units of 1/16 weight of oxygen. This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.
The discovery of isotopic oxygen in 1929 led to a divergence in atomic weight representation, with isotopically weighted oxygen (chemistry) and pure
^{16}O (physics) bases both used as the basis for the atomic mass unit (amu). The inevitable divergence could result in errors in computations, and was thus unwieldy. The reference was changed to
carbon12Carbon12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
in 1961 and a new symbol "u" replaced the now deprecated "amu".

The current unit is referred to as the "unified atomic mass unit" u. The choice of carbon12 was used to minimise further divergence with prior literature.
Terminology
The unified atomic mass unit and the dalton are different names for the same unit of measure. Since the dalton was first introduced, there has been a gradual change towards using it in preference to the unified atomic mass unit.
 In 1993, the International Union of Pure and Applied Chemistry approved the use of the dalton with the qualification that the GCPM had not given its approval.
 In 2003 the Consultative Committee for Units, part of the CIPM, recommended a preference for the usage of the "dalton" over the "unified atomic mass unit" as it "is shorter and works better with prefixes".
 In 2005, the International Union of Pure and Applied Physics endorsed the use of the dalton as an alternative to the unified atomic mass unit.
 In 2006, in the 8th edition of the formal definition of SI
Si, si, or SI may refer to : Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
, the CIPM cataloged the dalton alongside the unified atomic mass unit as a "NonSI units whose values in SI units must be obtained experimentally: Units accepted for use with the SI". The definition also noted that "The dalton is often combined with SI prefixes ..."
 In 2009, when the International Organization for Standardization published updated versions of ISO 80000, it gave mixed messages as to whether or not the unified atomic mass unit had been deprecated: ISO ISO 800001:2009 (General), identified the dalton as having "earlier [been] called the unified atomic mass unit u", but ISO 8000010:2009 (atomic and nuclear physics) catalogued both as being alternatives for each other.
 The 2010 version of the Oxford University Press style guide for authors in life sciences gave the following guidance "Use the Système international d'unités (SI) wherever possible ... The Dalton (Da) or more conveniently the kDa is a permitted nonSI unit for molecular mass or mass of a particular band in a separating gel." At the same time, the author guidelines for the journal "Rapid Communications in Mass Spectrometry" stated "The Dalton (Da) is a unit of mass normally used for the molecular weight ... use of the Da in place of the u has become commonplace in the mass spectrometry literature ... The "atomic mass unit", abbreviated "amu", is an archaic unit".
Relationship to SI
The definition of the
moleThe mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
, an
SI base unitThe International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...
, was accepted by the CGPM in 1971 as:
 The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol".
 When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
The definition of the mole also determines the value of the universal constant that relates the number of entities to amount of substance for any sample. This constant is called the Avogadro constant, symbol
N_{A} or
L, and is equal to entities per mole.
Given that the unified atomic mass unit is one twelfth the mass of one atom of carbon 12, meaning the mass of such an atom is 12u, it follows that there are
N_{A} atoms of carbon 12 in 0.012 kg of carbon 12. This can be expressed mathematically as
 N_{A}(12u) = 0.012 kg/mol, or
 N_{A}u = 0.001 kg/mol
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