| Atomic Mass Unit |
Article Index for Atomic |
Website Links For Atomic |
Information AboutAtomic Mass Unit |
| CATEGORIES ABOUT ATOMIC MASS UNIT | |
| nuclear chemistry | |
| units of mass | |
|
:1 u = 1/''N''A Gram = 1/ (1000 ''N''A) Kg (where ''N''A is Avogadro's Number ) :1 u ≈ 1.660538782(83) × 10−27 kg ≈ 931.494028(23) MeV/c2 See 1 E-27 Kg for a list of objects which have a mass of about 1 u. The symbol amu for '''atomic mass unit''' can sometimes still be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied. In Biochemistry and Molecular Biology literature (particularly in reference to Protein s), the term "dalton" is used, with the symbol Da. Because proteins are large Molecule s, they are typically referred to in kilodaltons, or "kDa", with one kilodalton being equal to 1000 daltons. The unified atomic mass unit, or dalton, is not an SI unit of mass, although it is accepted for use with SI under either name. The unit is convenient because one has an atomic weight of 35.45 u because it is composed of 76% 35Cl (34.96 u) and 24% 37Cl (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). 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 Gram . For example, the molecular mass of a Water molecule containing one 16O isotope and two 1H isotopes is 18.0106 u, and this means that one mole of this monoisotopic water has a mass of 18.0106 grams. Water and most molecules consist of a mixture of molecular masses due to naturally occurring isotopes. For this reason these sort of comparisons are more meaningful and practical using Molar Mass es 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 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. Francis Aston , inventor of the mass spectrometer, later used of the mass of one atom of Oxygen -16 as his unit. Before 1961 , the ''physical atomic mass unit'' (amu) was defined as of the mass of one atom of oxygen-16, while the ''chemical atomic mass unit'' (amu) was defined as of the ''average'' mass of an oxygen atom (taking the natural abundance of the different oxygen Isotope s 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). SEE ALSO EXTERNAL LINKS |
|
|