Dalton (unit)

The unified atomic mass unit or atomic mass unit (u), or dalton (Da) or, sometimes, universal mass unit (u), is a unit of mass used to express atomic and molecular masses. It is the approximate mass of a hydrogen atom, a proton, or a neutron. Accordingly the atomic mass of any atom is approximately the mass number (number of protons plus neutrons) times the atomic mass unit; the difference is the mass defect.

Values of 1 u	Units
1.660 538 782(83)×10−24	g
1.660 538 782(83)×10−27	kg
931.494 028(23)×106	eV/c2
For details, see below.

Definition

The atomic mass unit (u) is one twelfth of the mass of an isolated atom of carbon-12 () at rest and in its ground state.

${\rm {u}}={\frac {1}{12}}\ m(^{12}{\rm {C)}}$

In other words, "A single atom of carbon-12 has a mass of 12 u exactly, by definition."

The unified atomic mass unit, or dalton, is not an International System of Units (SI) unit of mass, but it is accepted for use with SI under either name.

The unit is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains n nucleons will have a mass approximately equal to n u. (The reason is that a ¹²
C
atom contains 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison. The mass of the electron is approximately 1/1836 of the mass of the proton.) This is an approximation, since it does not account for the mass contained in the binding energy of an atom's nucleus; this binding energy mass is not a fixed fraction of an atom's total mass. The differences which result from nuclear binding are generally less than 0.01 u, however. Chemical element masses, as expressed in u, would therefore all be close to whole number values (within 2% and usually within 1%) were it not for the fact that atomic masses of chemical elements are averaged values of the various stable isotope masses in the abundances which they naturally occur. For example, chlorine has an atomic mass of 35.45 u because it is composed of 76% ³⁵
Cl
(34.96 u) and 24% ³⁷
Cl
(36.97 u).

Another reason the unit is used is that it is experimentally much easier and more precise to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Masses are compared with a mass spectrometer (see below).

In biochemistry and molecular biology, when talking about mass of molecules, the term "dalton" is used, with the symbol Da. Because proteins are large molecules, their masses are often in kilodaltons, where one kilodalton is 1000 daltons.

The symbol amu is sometimes still used, mostly by error. "amu" is the symbol for two older units (confusingly both having the symbol amu) that are very similar but not equal to the modern atomic mass unit (see History).

Relation to SI units

Given that M_u is the Molar mass constant and N_A is the Avogadro constant, the relationship between the atomic mass unit (u) and the gram (g) is

$1\ {\textrm {u}}=1\ {\frac {\text{gram}}{\text{mole (of particles)}}}={\frac {M_{\rm {u}}}{N_{\rm {A}}}}={\frac {1\ {\frac {\text{gram}}{\text{mole}}}}{6.022\,141\,79(30)\times 10^{23}\ {\frac {\text{particles}}{\text{mole}}}}}=1.660\,538\,782(83)\times 10^{-24}\ {\frac {\text{gram}}{\text{particle}}},$

or

$1\ {\rm {u}}=1.660\,538\,782(83)\times 10^{-24}\ {\rm {g}}$

and

$1\ {\rm {g}}=6.022\,141\,79(30)\times 10^{23}\ {\rm {u}}$.

Avogadro's number (N_A) and the mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1 g. For example, the molecular mass of a water molecule containing one ¹⁶
O
atom and two ¹
H
atoms is 18.0106 u, and this means that one mole of this monoisotopic water has a mass of 18.0106 g. Water and most molecules consist of a mixture of molecular masses due to naturally occurring isotopes. For this reason these sorts of comparisons are more meaningful and practical using molar masses which are generally expressed in g/mol, not u. In other words the one-to-one relationship between daltons and g/mol is true but in order to be used accurately for any practical purpose any calculations must be made with isotopically pure substances or involve much more complicated statistical averaging of multiple isotopic compositions.

History

The chemist John Dalton was the first to suggest the mass of one atom of hydrogen as the atomic mass unit, and indeed in the early 19th century Prout's hypothesis (by William Prout) was that hydrogen atoms were the only true functional unit, though this was later disproven by more careful measurements. Francis Aston, inventor of the mass spectrometer, later used 1⁄16 of the mass of one atom of oxygen-16 as his unit, which was used in physics until 1961.

Before 1961, separate conventions were used in chemistry and physics – the physical atomic mass unit (amu) was defined as 1⁄16 of the mass of one atom of oxygen-16, while the chemical atomic mass unit (amu) was defined as 1⁄16 of the average mass of an oxygen atom (taking the natural abundance of the different oxygen isotopes into account). Both units are slightly smaller than the unified atomic mass unit, which was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961. Hence, before 1961 physicists as well as chemists used the symbol amu for their respective (and slightly different) atomic mass units. One still sometimes finds this usage in the scientific literature today. However, the accepted standard is now the unified atomic mass unit (symbol u), with: 1 u = 1.000 317 9 amu (physical scale) = 1.000 043 amu (chemical scale). Since 1961, by definition the unified atomic mass unit is equal to one-twelfth of the mass of a carbon-12 atom.

Atomic mass units (u) or daltons (Da)?

When dealing with atoms or simple molecules, it makes little difference as to whether one uses atomic mass units or daltons. However, when one is dealing with complex molecules (molecules having an atomic mass of over 1000 Da), it often makes sense to express the atomic weight in terms of kilodaltons (kDa). Likewise, when dealing with small changes (such as in nuclear physics), the use of millidaltons (mDa) or microdaltons (µDa) often provides a more readable text than using atomic mass units (where SI prefixes are not available). The International System of Quantities (ISO 80000-1:2009) favours dalton in all cases and deprecates the unified atomic mass unit.

See also

• Mass-to-charge ratio

External links

• Accepted value of 1 u as of 2006
• atomic mass unit