Atomic mass

as atomic mass (A) (English. atomic), in former times atomic weight ( English measured. atomic weight), one designates the mass of atoms of chemical elements.

It becomes between relative atomic mass (A r) (without unit) and absolute atomic mass, indicated with respect to kg, g or u differentiated.

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to meaning

from the relative atomic masses, the molecule masses calculable from it and on the basis the mol masses derived from it leave themselvesthe measurement and Volumenverhältnisse of the materials taken part in a chemical reaction compute.


the first table with relative atomic masses was published 1805 by John Dalton. It received it on the basis the mass ratios with chemical reactions, whereby it thatlightest atom, the hydrogen atom, when absolute dimensions selected, which were specified thereby arbitrarily to the value 1 (see Dalton).

Later the computation of the relative atomic and molecule masses for gaseous elements and connections on the basis of Avogadro's law took place, i.e. by weighing a well-known gas volume out, then also with the help of the Faraday's laws.

Later oxygen than absolute dimensions was taken and assigned to it arbitrarily the mass 16 (Jean Servais Stas, 1865).

Since the decision thatAtomic mass commission of the IUPAC of 1961 serves the carbon isotope 12 C as reference basis with the mass of 12. The relative atomic mass indicates, the mass of the respective atom than 1/12 of the mass of this carbon isotope is how often larger, there 12C-atom 12 nucleons (core components), more exactly 6 protons and 6 neutrons, contains. Since both nucleons exhibit a very similar mass, the atomic mass of an isotope almost corresponds to the number of contained nucleons (also mass number called). The small deviation becomes by thatMass difference between proton and neutron and the atomic mass defect cause.

The following table points some relative atomic masses in dependence to the three different absolute dimensions:

related to H = 1 related to O = 16 related to 12 C = 12
Hydrogen 1.000 1.008 1.008
chlorine 35.175 35.457 35.453
oxygen 15.872 16.000 15.999
nitrogen 13.896 14.008 14.007
carbon 11.916 12.011 12.011

absolute atomic mass

the absolute mass of an atom lies within the range of 10,-27 kg (0.000 000,000000,000,000,000,000,001 kg). By definition the twelfth part of a mol of the carbon isotope has 12 C a mass of 1 G. The absolute atomic mass is thus referred sometimes to gram (also Grammatom one calls). Due to that very muchsmall numerical values frequently the atomic Masseneinheit u is used, which likewise corresponds to 1/12 of the atomic mass of the carbon isotope 12 C. The numerical value of the mass of a particle in u and the numerical value of the mass of 1 mol of this particle in g are therefore identically.

average atomic mass

exact atomic masses today with the mass spectrometer are determined. The mass of the individual isotopes can be determined very precisely. For the determination of the relative atomic masses of the elements then still the abundance ratio must be determined.

For elements occurring on earth for purposes of chemistry the average atomic mass of the natural isotope mixture is indicated in the earth's crust; in special cases the origin of the isotope mixture must be considered. For purposes of physics is the atomic mass of the individual isotopemore interesting.

Further examples of the relative atomic masses of some chemical elements:


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