Isotopes in Nature

Atomic nuclei are composed mainly of protons, with a mass of 1.67262 X 10 g, or 1.007276 atomic mass units (amu, relative to = 12.000000), and neutrons, with a mass of 1.008665 amu (see appendix 1 for a complete energy list of nuclear properties). 

The term isotope was coined by Soddy (1914) to define two or more substances of different masses occupying the same position in the periodic chart of the elements. Soddy s hypothesis was adopted to explain apparent anomalies in the relative positions of three couples of elements (Ar-K, Co-Ni, and Te-I) in the periodic chart. For instance, potassium is present in nature with three isotopes with masses of 39, 40, and 41, respectively, in the following proportions = 93.26, = 0.01, and = 6.73. Because the pro- 

All stable nuchdes fall above the N = Z line, with the exception of H and He. Up to jN, the number of neutrons does not exceed the number of protons by more than 1. Above the stable nuclides diverge from the N/Z = 1 line upward, as a result of a progressive increase in the proportion of neutrons. 

Hydrogen atoms and part of He are believed to have been created during the Big Bang by proton-electron combinations. Most nuclides lighter than iron were created by nuclear fusion reactions in stellar interiors (cf table 11.1). Nuclides heavier than the Fe-group elements (V, Cr, Mn, Fe, Co, Ni) were formed by neutron capture on Fe-group seed nuclei. Two types of neutron capture are possible slow (s-process) and rapid (r-process). 

The lower chain in figure exemplifies the r-process several neutrons are added in a rapid sequence until a highly unstable Z-N combination is attained (crosshatched boxes), j8 decay then takes place. 

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Coupling constants are routinely used to determine the side-chain conformation of amino acids in peptides and proteins. Whereas proteins nowadays are almost exclusively studied as C- and N-labeled isotopomers, peptides usually have these isotopes in natural abundance, i.e. the magnetically active heteronuclei are highly diluted. Most amino acids contain a methylene group at the ji-position for which the X angle is determined by the conformation of the Ca—Cp bond. Two vicinal Jhh coupling constants can be measured Ha to and H to Usually... 

Figure 10.2 The radioactive stability of the elements. The x axis is proton number (up to Z = 83, bismuth), the y axis the neutron number (N). Stable isotopes are shown in black and radioactive isotopes in grey, indicating the relative excess of radioactive isotopes over stable isotopes in nature, and the fact that as proton number increases, the neutron number has to increase faster to maintain stability. The basic data for this figure are given in Appendix VI.

Until it was known that the half-life of Tc is much shorter than 10 years Herr looked for technetiimi-98 in a munber of minerals from Norway and South Africa by neutron activation analysis. This most sensitive method was also used by Alperovitch et al. and Anders et al. to detect this isotope in nature. According to the nuclear reaction... 

Out of the numerous possibilities to fractionate oxygen isotopes in nature, the following are of special significance. 

Because selenium to some extent is chemically similar to sulfur, one might expect to find some analogous fractionations of selenium isotopes in nature. Six stable selenium isotopes are known with the following abundances (Coplen et al. 2002)... 

Isotope % In Natural Thorlurn MaJ or Mev Alpha Energies (abundances) Half-Life... 

In addition, one of the main features of mass spectrometry is, and this is the major advantage in comparison to other atomic and molecular non-mass spectrometric techniques, that it offers the possibility of determining isotope ratios and abundances of isotopes with high precision and accuracy in all types of samples (in solid, liquid and gaseous materials as well). Isotope ratio measurements have applied increasingly for stable isotopes in nature, especially for investigating... 

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

The percentage of this isotope in naturally occurring molecules containing this element. 

Nielsen H. (1978) Sulfur isotopes in nature. In Handbook of Geochemistry (ed. K.H. Wedepohl), Part 16B. Springer Verlag, Berlin. 

What is the ratio of effusive velocities of 235UF6 and 238UF6 What would be the fraction of the 235U isotopomer effusing from a mixture of hexafluorides that contains 0.50% 235U (the percentage of the isotope in natural abundance) ... 

Be is not a stable isotope despite its large neutron separation energy, because it can beta decay to the more stable isobar 7Li (see under Mass excess in Glossary). As a resultno 7Be exists on Earth or in the meteorites, except by transient production by cosmic rays, and none has been seen (yet) in stars. But it is stable against breaking up into nuclear particles (as opposed to beta decay) and is an observable isotope in nature in two ways. 

The discovery in 1973 that refractory minerals found in meteorites are enriched in the l60 isotope launched a new era of cosmic chemical memory of nucleosynthesis. Though only 5%, the excess showed that not all rocks in the solar system were assembled from well mixed gas of the solar system. It suggested that presolar materials also can be found. The study of isotopes in natural history was accelerated by this pivotal discovery. 

C1 is the 38th most abundant isotope in nature. 

Carbon has three isotopes in nature 12C—common and stable 13C—rare and stable and 14C—very rare and radioactive. The heavy carbon isotope, 14C, is unstable and decays radioactively into 14N, emitting a beta ([ > ) particle that can be measured in specialized laboratories. The half-life of 14C is 5730 years. The above information can be summarized in the following way ... 

Chlorine has three isotopes in nature 35C1—common and stable 36C1—very rare and radioactive, with a 301,000-year half-life and 37C1—less common and stable. Chlorine-36 concentrations are commonly expressed in units of 107 atoms/1 of water. The measurement is performed in specialized laboratories with dedicated accelerators. Measurements can be done on 1-1 water samples. 

Sakai, H. Fractionation of sulphur isotopes in nature. Geochim. et Cosmochim. Acta 12, 150(1957). 

A nuclide may be specified by attaching the mass number as a left superscript to the symbol for the element. The atomic number may also be attached as a left subscript, if desired, although this is rarely done. If no left superscript is attached, the symbol is read as including all isotopes in natural abundance. 

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