Atomic mass unit
The atomic mass unit ( unit symbol: u for unified atomic mass unit, amu outdated for atomic mass unit ) is a unit of mass. It is used when specifying atomic and molecular masses. Its value is set to 1/12 the mass of an atom of carbon isotope 12C.
A mass of 1 u is approximately equal to the mass of a proton or a neutron. Therefore, the numerical value of the atomic mass in u corresponds approximately to the mass number or nucleon number, ie the number of severe core building blocks (protons and neutrons) of the atom.
In biochemistry and in the U.S. in organic chemistry, the atomic mass unit is also referred to as Dalton ( unit symbol: Da), named after the English naturalist John Dalton. In the German translation of the brochure of the International Bureau of Weights and Measures, the Dalton and the ( unified ) atomic mass unit are used synonymously. In the legal provisions of the EU Directive 80/181/EEC for the EU member states and in the Federal Law on Metrology in Switzerland the term " Dalton " comes but not before. Even if Dalton can be considered as a special name for the atomic mass unit, the name Dalton is neither legally nor DIN to standard. The atomic mass unit is approved for use with the International System of Units ( SI) and a legal unit.
Definition
Today's value (since 1961)
1 u is 1/ 12 of the mass of an isolated atom of the carbon isotope 12C in the ground state, ie
Thus arises as conversion into the SI unit kilogram:
Since the core of the 12C atom contains 12 nucleons, the unit u is approximately equal to the mass of a nucleon.
Value up to and including 1960
An atomic mass unit corresponded to 1/16 the mass of an oxygen atom.
The difference between the " chemical " definition (based on the natural isotopic mixture of oxygen ) and the " physical " definition (based on the isotope 16O ) was the occasion to introduce the modern, unified definition. Between the new and the old values of the unit the relation
The difference between the old and the current physical definition due to the mass defect that is higher than 12C at 16O.
Multiples and parts
Both for the atomic mass unit as well as for the use of the Dalton intentions of decimal multiples and parts is permitted. Commonly used are the kilodaltons, 1 kDa = 1000 Da, and the Mega Dalton, 1 MDa = 1 million Da.
Examples
- A carbon atom of the isotope 12C has been defined as the mass 12 and
- One molecule of the active ingredient known acetylsalicylic acid ( aspirin) has a mass of 180.16 u
- One molecule of the small peptide hormone insulin has a mass of 5808 u
- One molecule of the protein actin (one of the most common proteins in eukaryotes ) has a mass of about 42,000 and
- A hydrogen atom of the isotope 1H has the mass of 1.007 825 u 0
Dalton, the unit is also used in the membrane filtration. It is a measure of the separation capability of a membrane here. Making a comparison between Dalton and pore size, enter the following reference values for specific filtration process:
Migrate in the analysis of protein mixtures by SDS-PAGE, wherein the SDS - denatured proteins in an electric field as defined by a gel with pore diameter, kDa is used as unit. This is not entirely unproblematic, since kDa is a unit of mass, not weight unit. You try to work around this problem by the concept; introduced (relative molecular mass relative molecular mass ), but still frequently and wrongly weight as molecular (molecular weight) in publications or for SDS -PAGE standard proteins (Molecular Weight Markers ) can be found.
Relationship to grams and moles
The mole, unit of amount of substance, the atomic mass unit linked to the gram. Contains one mole
Particles, and this number is, the Avogadro constant, is the conversion factor between grams ( g) and the atomic mass unit (u). Therefore, the molar mass in g / mol is numerically equal to the molecular mass and