The unified atomic mass unit or dalton is the standard unit that is used for indicating
mass on an atomic or molecular scale. One unified atomic mass unit is approximately
the mass of one nucleon and is equivalent to 1 g/mol. It is defined as one twelfth
of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground
state, and has a value of 1.660538921(73)×10−27 kg. The CIPM has categorised it as a non-SI unit
accepted for use with the SI, and whose value in SI units must be obtained experimentally.
The amu without the "unified" prefix is technically an obsolete unit based on oxygen, which was
replaced in 1961. However, some sources still use the amu but now define it in the same
way as u. In this sense, most uses of atomic mass units or amu today actually refer to
unified atomic mass units or u. For standardization a specific atomic nucleus had to be chosen
because average mass of a nucleon depends on the count of the nucleons in the atomic
nucleus due to mass defect. This is also why the mass of a proton by itself is more than
1 u. Atomic mass unit does not stand for the unit
of mass in the atomic units system, which is rather me.
History The relative atomic mass scale has traditionally
been a relative scale, that is without an explicit unit, with the first relative atomic
mass basis suggested by John Dalton in 1803 as 1H. Despite the initial mass of 1H being
used as the natural unit for relative atomic mass, it was suggested by Wilhelm Ostwald
that relative atomic mass would be best expressed in terms of units of 1/16 mass 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 relative atomic mass representation, with isotopically weighted oxygen given a
value of exactly 16 atomic mass units in chemistry, while pure 16O was given the mass value of
exactly 16 amu in physics. The divergence of these values could result
in errors in computations, and was unwieldy. The chemistry amu, based on the relative atomic
mass of natural oxygen, was about 1.000282 more massive than the physics amu, based on
pure isotopic 16O. For these and other reasons, the reference
standard for both physics and chemistry was changed to carbon-12 in 1961. The choice of
carbon-12 was made to minimise further divergence with prior literature. The new and current
unit was referred to as the "unified atomic mass unit" u. and given a new symbol "u,"
which replaced the now deprecated "amu" that had been connected to the old oxygen-based
system. However, modern sources often still use "amu"
in place of "u" and define "amu" in terms of carbon-12. In general, "amu" probably does
not refer to the old oxygen standard amu, unless the material is older than the 1960s.
The "unified atomic mass unit" u was defined as:
Terminology The unified atomic mass unit and the dalton
are different names for the same unit of measure. As with other unit names such as watt and
newton, "dalton" is not capitalized in English, but its symbol Da is capitalized. With the
introduction of the name "dalton", there has been a gradual change towards using that name
in preference to the name "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, the CIPM cataloged the dalton alongside the unified atomic mass unit as
a "Non-SI 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 80000-1:2009,
identified the dalton as having "earlier [been] called the unified atomic mass unit u", but
ISO 80000-10:2009 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 wherever possible
... The dalton or more conveniently the kDa is a permitted non-SI 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 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". In 2012, in response to the proposed redefinition
of the kilogram, it was proposed that the dalton be redefined as being 0.001/NA kg,
thereby breaking the link with 12C. This would result in the dalton and the atomic mass unit
having slightly different definitions, but the suggestion is that the older unit should
be superseded by the "new" dalton. Relationship to SI
The definition of the mole, an SI base unit, 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 NA or L, and is equal to 6.02214129(27)×1023 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 12 u,
it follows that there are NA atoms of carbon-12 in 0.012 kg of carbon-12. This can be expressed
mathematically as NA = 0.012 kg/mol, or
NA u = 0.001 kg/mol Masses of proteins are often expressed in
daltons. For example, a protein with a molecular weight of 64,000 g mol−1 has a mass of 64,000
daltons or 64 kDa in short. Examples
A hydrogen-1 atom has a mass of 1.007 825 0 u.
By definition, a carbon-12 atom has a mass of 12 u.
A molecule of acetylsalicylic acid has a mass of 180.16 u.
Titin, the largest known protein, has an atomic mass of 3-3.7 megadaltons.
See also Kendrick mass
Mass-to-charge ratio Atomic mass constant
Notes and references
External links atomic mass unit at sizes.com