Talk:Dalton

Question

I'm a little puzzled. In one place (http://en.wikipedia.org/wiki/Dalton#Unit_of_mass), the Dalton is defined thus:

Whereas the unified atomic mass unit is defined as 1/12 of the mass of a single atom of 12C, the dalton was defined as 1/16 of the mass of an atom of oxygen (which turned out to be a mixture of 16O, 17O and 18O in a 3150:1:5 ratio with mean atomic mass number of 16,0035).

But in another place (http://en.wikipedia.org/wiki/Atomic_mass_unit) it is defined thus:

The atomic mass unit (amu), unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. It is defined to be 1/12 of the mass of one atom of Carbon-12.

Which is correct?

They're really both right. Dalton used the hydrogen molecule (H2) as his unit, and later (by 1900 or so) the two "competing" systems were H = 1 and O = 16. In part because of the oxygen isotope mix, the switch to C = 12 occurred in 1961.

The 1913 Webster says « Microcrith (Chem.): The weight of the half hydrogen molecule, or of the hydrogen atom, at one time taken as the standard in comparing the atomic weights of the elements; thus, an atom of oxygen weighs sixteen microcriths. This unit is no longer used, and has been replaced by the dalton, which is of approximately the same value. See molecular weight and Crith. --J. P. Cooke. [1913 Webster +PJC] »

The SI document simply states that the (C-12) amu is called the Dalton in the biochemistry domain. Ascribing the dalton to O-16 seems apocryphal.

According to the International Union of Pure and Applied Chemistry (IUPAC) Compendium of Chemical Terminology, the dalton is a « Non-SI unit of mass (symbol Da), equal to the unified atomic mass unit (atomic mass constant). It is often used in biochemistry and molecular biology although it was never approved by the Conférence générale des poids et mesures. G. B. 111; 1996, 68, 968 IUPAC Compendium of Chemical Terminology » (emphasis added).

Urhixidur 05:15, 2005 Feb 5 (UTC)