Modern technologies allow very accurate measurements of atomic weights and isotope ratios of elements. This is of high importance in solving research problems in others fields. For example, accurate measurement of isotope ratios of carbon in the sample can be used for assessment of of quality of food. In doping investigations distinction between the isotope patterns can also be useful – the distribution of isotopes in the pharmaceutical testosterone and in the testosterone produced by the human body is different. At the present time in the periodic table there are some values of atomic weights that were determined more than a century ago. Nevertheless, recently the data obtained by mass spectrometry has been useful in decreasing the measurement uncertainty of atomic weights.
For example, in the case of carbon, the highest abundance of the 13C isotope is observed in the matter found in the deep sea. Carbon obtained from such matter has an atomic weight value of 12.011505. The uncertainty in this atomic weight value due to the uncertainty in the delta-value determination is 0.000003.
This is an example of the situation encountered with many elements that the atomic weight depends on a place of extraction of the element from the Earth`s crust. The International Union of Pure and Applied Chemistry (IUPAC) have recently published the new table of Standard Atomic Weights with updated atomic weight values for ten elements: hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium. The new values more precisely reflect the isotopic distribution of these elements in the Nature.
The atomic weights are expressed as intervals, having upper and lower limits. For example, for nitrogen the value of atomic weight is now 14.0067 and according to new approach by IUPAC is presented in the form of an interval: from 14.00643 to 14.00728. This reflects the dependence of the relative atomic weight on the source of the element. IUPAC hope to publish a new periodic table in the near future (Image: Wikipedia).
