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 54 × 10⁻²⁴ gram. 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 and its use is not sanctioned by the CGPM. 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.

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. 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.Consultative Committee for Units (CCU).
Report of the 15th meeting (17-18 April 2003) to the International Committee for Weights and Measures.
Reports of CCU meetings are now published only on the BIPM's web site, www.bipm.org. Accessed 22 May 2007.

See section 4.16 page 9.

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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