Mononuclidic elements

Mononuclidic elements are chemical elements that occur in nature and consist of only one nuclide isotope they are sometimes also called monoisotopic. They are listed in Table C.l. 

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In summary, as a consequence of the factors considered in this and the preceding section, the atomic weights of only the 20 mononuclidic elements can be regarded as constants of nature . For all other elements variability in atomic weight is potentially possible and in several instances is known to occur to an extent which affects the reliability of quantitative results of even modest precision. 

Figure lA. Periodic table depicting utility of positive ion TIMS (mononuclidic elements have not been considered), a. Elements for which isotope dilution technology has been developed at NBS b, additional elements for which isotope ratio or isotope dilution measurements have been reported ( ) and c, elements for which positive ion TIMS is potentially applicable. 

Similarly, natural thorium is a mononuclidic element, i.e., it has only one radioactive isotope thorium-232, parent radionuclide of the natural decay chain (4n), therefore, for a naturally occurring radioactive material containing a mass fraction of natural thorium, the specific activity of the parent radionuclide is given by ... 

Fundamental atomic and physical properties of the alkali metals are given in Tables 1, 2, and 3. The elements are characterized by having electron configurations each with a single s orbital electron outside a noble gas core (see Table 1). Sodium and cesium are mononuclidic so that their relative atomic masses are known extremely accurately in effect, the same can be written for potassium and rubidium since their isotopes (of which there are three and two, respectively) have... 

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