Relative atomic mass - Definition

08-04  Source: Network gathering  Views:2 


Relative atomic mass - Definition

Even atoms have mass.

Relative atomic mass - the one understands thereunder

  • The atom of ordinary hydrogen has a tiny mass, namely around 1.7 * 10 -27 kg. The masses of the heavy atoms are on the order of 10 -25 kg. To avoid calculating with such small numbers, it is convenient to introduce instead of the atomic mass m a, the dimensionless relative atomic mass A r. The subscript, small "r" is intended for are "relatively" here.
  • But for relative definitions one needs first a kind of mass unit, to which one refers. For this, the so-called. Atomic mass unit (symbol "u") is selected, (ie the "conventional" carbon) corresponds to the 12th part of the rest mass of a neutral carbon atom with nucleons 12th Accordingly, m u = applies 1/12 m a (12 C) = 1.66053 * 10 -27 kg = 1 u (an atomic unit or "unit").
  • The relative atomic mass of any nucleus is then given by A r = m a / m u. This ratio is often inaccurately referred to as "atomic weight".

determine atomic masses - so follow the definition

  • By this definition, the relative atomic mass of Bezugsnuklids 12-C has (with 6 neutrons and 6 protons, ie 12 Kernnukleonen) the value of A r (12-C) = 12th
  • Atomic masses can be determined by individual nuclides with a fixed number of protons and neutrons. but also of elements with (slightly different stable) isotopes. Because even the first mass spectroscopic investigations showed that for atoms having the same atomic (ie, the same number of protons) usually a whole series (stable) isotopes exist with different masses. They differ in the number of neutral core particle, the neutron. From abundance ratio of each isotope ultimately depends on the atomic weight of an element. Therefore, it is not an integer.
  • Relative atomic masses of the various elements can be found in tabular works or the Internet. Always close the masses of the electron shells A (ie refer to neutral atoms). These are, however, in view of an electron mass m e = 9.1 * 10 -31 kg extremely small and need - are not further considered - except for accurate mass calculations.
  • Three more examples of such tables may be mentioned. Thus, the relative atomic mass of lead 207.19, the oxygen and the iron 15.9994 55.847.
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