Abundances of Elements
KeywordsChemical composition Nucleosynthesis Nuclide
The relative amount (or fraction) of a given nuclide in a sample of matter is called the abundance of that nuclide. It can be expressed either in absolute terms (i.e., with respect to the total amount of matter in the sample) or in relative terms (with respect to the amount of some key element, e.g., the most abundant one, in the sample). Similarities and differences in the elemental and isotopic composition of stars and galaxies are key ingredients for understanding their origin and evolution.
The composition of remote objects (the Sun, stars, interstellar gas, and galaxies) is determined through spectroscopy, which usually allows the determination of elemental abundances; in rare cases, particularly for interstellar clouds, some isotopic abundances may be determined in those objects. For Earth, lunar, and meteoritic samples, nuclear mass spectroscopy allows precise determination of most isotopic abundances; this is also the case for cosmic rays, albeit only for the most abundant nuclides at present. Hydrogen (H) being the most abundant element in the Universe, spectroscopists express the abundance of element i as the number ratio of its nuclei with respect to those of H: n i = N i /N H , and they use a scale where log(N H ) = 12. In the meteoritics community, the silicon scale of log(N Si ) = 6 is used. Theoreticians use the mass fraction X i = N i A i / ∑N j A j , where Aj is the mass number of nuclide j; obviously, ∑X i = 1. Conversion of mass fractions to abundances by number requires use of the quantity Y i = X i /A i called the mole fraction (notice that ∑Y i ≠ 1).