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Reference.org
Dalton (unit)
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<p> The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of <a href="/facts/Mass/HHdc8XmF">mass</a> defined as 1/12 of the mass of an <a href="/facts/Bound_state/NndW5JQl">unbound</a> neutral atom of <a href="/facts/Carbon-12/nJYECzEr">carbon-12</a> in its nuclear and electronic <a href="/facts/Ground_state/ML7cfCug">ground state</a> and <a href="/facts/Invariant_mass/wNGXNBW6">at rest</a>. It is a <a href="/facts/Non-SI_units_mentioned_in_the_SI/uBgEpKMP">non-SI unit accepted for use with SI</a>. The word "unified" emphasizes that the definition was accepted by both <a href="/facts/IUPAP/ah3BodlS">IUPAP</a> and <a href="/facts/IUPAC/2AV6myCK">IUPAC</a>. The atomic mass constant, denoted <i>m</i>u, is defined identically. Expressed in terms of <i>m</i>a(12C), the <a href="/facts/Atomic_mass/J1FJY9MV">atomic mass</a> of carbon-12: <i>m</i>u = <i>m</i>a(12C)/12 = 1 Da. Its value in <a href="/facts/International_System_of_Units/S7YLglNz">SI units</a> is an experimentally determined quantity. The 2022 <a href="/facts/CODATA/68o8u2vc">CODATA</a> recommended value of the atomic mass constant expressed in the SI base unit <a href="/facts/Kilogram/lU2zbPk8">kilogram</a> is:</p><blockquote><p><i>m</i>u = 1.66053906892(52)×10−27 kg.</p></blockquote><p> As of June 2025, the value given in the <a href="/facts/SI_Brochure/S7YLglNz">SI Brochure</a> is the 2018 CODATA recommended value:</p><blockquote><p>mu = 1.660539040(20)×10−27 kg.</p></blockquote><p> The value serves as a <a href="/facts/Conversion_of_units/BY9Az4t6">conversion factor</a> of mass from daltons to kilograms, which can easily be converted to <a href="/facts/Gram/CNAPqWKz">grams</a> and other metric units of mass. The <a href="/facts/2019_revision_of_the_SI/g0h07jCX">2019 revision of the SI</a> redefined the kilogram by fixing the value of the <a href="/facts/Planck_constant/qGHAe3qq">Planck constant</a> (<i>h</i>), improving the precision of the atomic mass constant expressed in SI units by anchoring it to fixed <a href="/facts/Physical_constant/uI2QvAHw">physical constants</a>. Although the dalton remains defined via carbon-12, the revision enhances traceability and accuracy in atomic mass measurements. </p><p>The <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a> is a unit of <a href="/facts/Amount_of_substance/tQVyNLCp">amount of substance</a> used in <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> and <a href="/facts/Physics/a7aH2y08">physics</a>, such that the mass of one mole of a substance expressed in <a href="/facts/Gram/CNAPqWKz">grams</a> (i.e., the <a href="/facts/Molar_mass/m2laJErl">molar mass</a> in g/mol or kg/kmol) is numerically equal to the average mass of an <a href="/facts/Molecular_entity/6Pcn5HDR">elementary entity</a> of the substance (<a href="/facts/Atom/69syB8dH">atom</a>, <a href="/facts/Molecule/N9ljSfFq">molecule</a>, or <a href="/facts/Formula_unit/P6LUge8g">formula unit</a>) expressed in daltons. For example, the average mass of one molecule of <a href="/facts/Water/0lkXnJPK">water</a> is about 18.0153 Da, and the mass of one mole of water is about 18.0153 g. A protein whose molecule has an average mass of 64 kDa would have a molar mass of 64 kg/mol. However, while this equality can be assumed for practical purposes, it is only approximate, because of the <a href="/facts/2019_revision_of_the_SI/g0h07jCX">2019 redefinition of the mole</a>. </p>