A very small unit of mass, about the mass of a hydrogen atom. Presently taken as an alternative name for the unified atomic mass unit, that is, one-twelfth of the mass of a carbon-12 atom, about 1.660 538 86 × 10⁻²⁷ kilogram. Symbol, Da, and previously sometimes D or d.

It is named for the English physical chemist John Dalton (1766 – 1844), who in 1803 proposed the modern idea of the atom and (though he didn't call it that) molecule.

In the first half of the 20ᵗʰ century the value of the dalton depended on the discipline in which it was used. Chemists used 1.660 24 × 10⁻²⁴ grams, while physicists used the value 1.659 79 × 10⁻²⁴ grams.

The dalton is not an SI unit. It has been approved, however, as late as 1993 by the International Union of Pure and Applied Chemistry,¹ as a special name for the atomic mass unit. Its use in that sense is particularly common in biochemistry and mass spectrometry, where it is used to describe the size of giant organic molecules, often in kilodaltons or megadaltons. The dalton appears to be inching (centimetering?) slowly towards acceptance as the name of the SI atomic mass unit (“kilo unified atomic mass unit” is such an awkward term), first alongside the unified atomic mass unit² and perhaps eventually replacing it.

Google's N-gram viewer shows the changes in the use of the word dalton in English publications between 1900 and 2000. Bear in mind that “dalton” includes the surname.

Leonard³ offers two redefinitions of the dalton. One is based on redefining the kilogram as a certain number of carbon-12 atoms, in which case the dalton's mass is the mass of a kilogram divided by twelve times that number. The CGPM has now rejected that approach. Alternatively and preferred by him, a redefinition of the kilogram based on assigning an exact value to Planck's constant also implies a definition of the dalton.

1. International Union of Pure and Applied Chemistry, Physical Chemistry Division.
Quantities, Units and Symbols in Physical Chemistry.
Oxford: Blackwell Scientific, 1993.

2. In the 2006 edition of the SI brochure:

The first three units, the non-SI units electronvolt, symbol eV, dalton or unified atomic mass unit, symbol Da or u, respectively, and the astronomical unit, symbol ua, have been accepted for use with the SI by the CIPM.

Bureau International des Poids et Mesures.
The International System of Units. 8th edition, 2006.
page 125.

We haven't been able to find documentation of the CIPM's approval. But in the meetings of the Consulative Committee on Units, the dalton was clearly accepted:

A request was made by Prof. Mills that the dalton (referred to in Table 7, footnote c) be included in the index.

Committee for Units (CCU) 14th Meeting (April 2001)

4.16 The dalton (symbol Da) is used as a name for the unified atomic mass unit. It is widely used by polymer chemists and biochemists (see footnote (c) in Table 7 of the current Brochure). The CCU acknowledged that the name Dalton has advantages over “unified atomic mass unit”: it is shorter and works better with prefixes. The “unification” between chemists and physicists which gave the unit its name is now of historic interest only. A decision was taken to introduce the dalton into Table 7, according it equal status with the unified atomic mass unit.

Following a question by Dr Taylor as to whether the CCU had the authority to make such a change, Prof. Mills agreed to draft a letter consulting IUPAP and IUPAC, to be reviewed by Drs Flowers and Dybkaer.

Consultative Committee for Units (CCU).
Report of the 15th meeting (17-18 April 2003) to the International Committee for Weights and Measures.
Page 9.

In the draft of Chapter 4 dated July 2004, sent by Prof. Mills for consideration by the members, Table 7 included the natural units of speed, action, mass, and time, and the atomic units of charge, mass, action, length, energy, and time, together with the experimental non-SI units the electronvolt, the dalton, the unified atomic mass unit, and the astronomical unit. Dr Capitaine said that a reference should be given for these four units and proposed to provide the most recent reference for the SI value of the astronomical unit.

Bureau International des Poids et Mesures
Consultative Committee for Units (CCU)
Report of the 16th meeting (13–14 May 2004)

3. B. P. Leonard.
On the role of the Avogadro constant in redefining SI units for mass and amount of substance.
Metrologia, volume 44, pages 82-86 (2007).

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