Chemical elements natural isotopic abundances

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and obser-vationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmo-chemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium. 

TABLE 5.4. Natural Isotopic Abundances of Widespread Chemical Elements... 

An average molecular mass (weight) of the polymer also needs to be defined, since components of various molecular masses (weights) M, are present in the polymer. The number of moles of species i in the polymer can be obtained from the typical formula ni = W] / Mi where W is the weight fraction of the component i", and M, is the molecular mass of species i . The masses of molecules or groups can be calculated using two different conventions. One convention considers the natural isotopic abundance of elements and takes their sum based on the compound chemical formula. For the masses of polymers, the first convention is typically used. The other convention considers only the masses of the most abundant isotope, which is useful for MS... 

Fractionation 1) Selective separation of chemical elements or isotopes through physical, chemical or biochemical processes. For example, the fractionation of carbon isotopes of the naturally occurring carbon isotopes -98.9% is C, 1.1% is and only 1 part in 10 % is In nature, however, a fractionation of this ratio occurs (e.g., photosynthesis results in an enrichment of relative to the other isotopes in most plant tissues). Based on thermodynamical laws, which show that the heavier isotope " C is twice as enriched as C, radiocarbon laboratories correct for the probable effects of fractionation. C can be measured in a sub-sample of the material to be dated and the C C ratio is then compared with a standard (PDB) and published as deviation from this standard. See also normalization. 2) In physical and chemical processes involving the isotopes of a particular element, the relative abundance of the isotopes may change between the initial substance (the substrate ) and its product. In isotope geochemistry, this change is commonly referred to as fractionation, (see also fractionation factor isotope effects). 

The chemical elements are uniquely distinguished by their atomic number, Z, which is the number of protons in an atom. According to the lUPAC terminology, the atom is the smallest part of an element with no net electric charge, which can enter into chemical combinations. Each neutral atom contains as many electrons as protons in its nucleus. Atoms of the same element with the same atomic number, but different number of neutrons are called isotopes. The isotopic abundance (isotope abundance or atom abundance) is the relative number of atoms of a particular isotope in a mixture of the isotopes of a chemical element, expressed as a fraction (%) of all the atoms of the element. The isotopic abundances of the elements vary depending on the source of materials. The natural isotopic abundance is the isotopic abundance in the element as found on Earth. Because of natural radioactivity, this quantity may not be well defined for some isotopes. 

When determining an NDP profile, it should be remembered that only the isotopic concentration is actually determined and that the elemental profile is inferred. While natural isotopic abundances are usually present, anthropogenic activities can severely distort isotopic ratios, especially for boron and lithium. Natural processes can also perturb isotopic proportion, however this is both rare and small in magnitude. Materials prepared by ion implantation will be isotopically different from natural materials. "B is more abundant (80.1%) and consequently more economical to implant than B. As a result, boron-implanted samples must be especially prepared for NDP analysis. Li is less abundant in nature and historically has been separated from natural lithium for nuclear applications. This provided a relatively inexpensive source of Li-depleted, i.e., purified, lithium available for chemical applications. 

Isotopes of an element are formed by the protons in its nucleus combining with various numbers of neutrons. Most natural isotopes are not radioactive, and the approximate pattern of peaks they give in a mass spectrum can be used to identify the presence of many elements. The ratio of abundances of isotopes for any one element, when measured accurately, can be used for a variety of analytical purposes, such as dating geological samples or gaining insights into chemical reaction mechanisms. 

This book presents a unified treatment of the chemistry of the elements. At present 112 elements are known, though not all occur in nature of the 92 elements from hydrogen to uranium all except technetium and promethium are found on earth and technetium has been detected in some stars. To these elements a further 20 have been added by artificial nuclear syntheses in the laboratory. Why are there only 90 elements in nature Why do they have their observed abundances and why do their individual isotopes occur with the particular relative abundances observed Indeed, we must also ask to what extent these isotopic abundances commonly vary in nature, thus causing variability in atomic weights and possibly jeopardizing the classical means of determining chemical composition and structure by chemical analysis. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

Stable isotopes serve as naturally occurring tracers that can provide much information about how chemical reactions proceed in nature, such as which reactants are consumed and at what temperature reactions occur. The stable isotopes of several of the lighter elements are sufficiently abundant and fractionate strongly enough to be of special usefulness. Foremost in importance are hydrogen, carbon, oxygen, and sulfur. 

Furthermore, isotope analysis is relevant for determining the atomic weight (Ar(E)) of elements. The Ar(E) is the average of all masses of all naturally occurring stable isotopes (taking into account the abundances of isotopes) of a chemical element (see Appendix I10). By consideration of the masses of isotopes (/ ,) and the known relative abundances of all stable isotopes (Xi) with i = 1 to n of a selected chemical element, the average atomic weight (Ar(E)) of this element can be calculated ... 

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