# Atomic mass unit

Unified atomic mass unit
(Dalton)
Unit systemPhysical constant
(Accepted for use with the SI)
Unit ofmass
Symbolu or Da
Named afterJohn Dalton
Conversions
1 u or Da in ...... is equal to ...
kg   1.660539040(20)×10−27
MeV/c2   931.4940954(57)
me   1822.888486192(53)

The unified atomic mass unit or dalton (symbol: u, or Da) is a standard unit of mass that quantifies mass on an atomic or molecular scale (atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.[1] 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 at rest,[2] and has a value of 1.660539040(20)×10−27 kg, or approximately 1.66 yoctograms.[3] 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.[2]

The atomic mass unit (amu) without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, many sources still use the term amu but now define it in the same way as u (i.e., based on carbon-12).[4][5] In this sense, most uses of the terms atomic mass units and amu, today, actually refer to unified atomic mass unit. For standardization, a specific atomic nucleus (carbon-12 vs. oxygen-16) had to be chosen because the 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 or neutron by itself is more than (and not equal to) 1 u.

The atomic mass unit is not the unit of mass in the atomic units system, which is rather the electron rest mass (me).

## History of amu

The standard atomic weight (or atomic weight) scale has traditionally been a relative value, that is without a unit, with the first relative atomic mass basis suggested by John Dalton in 1803 as 1H.[6] 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 (1903). This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.[6]

The discovery of isotopic oxygen in 1929 led to a divergence in relative atomic mass representation, with isotopically weighted oxygen (i.e., naturally occurring oxygen relative atomic mass) given a value of exactly 16 atomic mass units (amu) in chemistry, while pure 16O (oxygen-16) 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 (atomic weight) of natural oxygen (including the heavy naturally-occurring isotopes 17O and 18O), was about 1.000282 as massive as 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.[7] The choice of carbon-12 was made to minimise further divergence with prior literature.[6] The new and current unit was referred to as the unified atomic mass unit, u.[8] and given a new symbol, "u", which replaced the now deprecated "amu" that had been connected to the old oxygen-based system. The dalton (Da) is another name for the unified atomic mass unit.[9]

${\displaystyle 1\,\mathrm {u} =m_{\mathrm {u} }={\frac {1}{12}}m\left({}^{12}\mathrm {C} \right)}$

Despite this change, modern sources often still use the old term "amu" but define it as u (1/12 of the mass of a carbon-12 atom), as mentioned in the article's introduction. Therefore, in general, "amu" likely does not refer to the old oxygen standard unit, unless the source material originates from the 1960s or before.

Other Languages
العربية: وحدة كتل ذرية
Cymraeg: Uned Dalton
Esperanto: Atommasa unuo
Bahasa Indonesia: Satuan massa atom
norsk nynorsk: Atommasseeining
oʻzbekcha/ўзбекча: Massaning atom birligi
Plattdüütsch: Atomare Masseneenheit
Simple English: Atomic mass unit
slovenščina: Enota atomske mase
srpskohrvatski / српскохрватски: Jedinica atomske mase
svenska: Atommassenhet