solar system abundances of the chemical elements

solar system abundances of the chemical elements Relative abundances of the chemical elements in the solar system span some 12 orders of magnitude (Fig. 1). The two lightest elements, hydrogen (H) and helium (He), together constitute more than 99 per cent of the atoms and the mass of the solar system. At the opposite end of the abundance scale are the rarest heavy elements, such as tantalum (Ta), rhenium (Re), thorium (Th), and uranium (U). Even elements as familiar as oxygen (O), iron (Fe), copper (Cu), and silver (Ag) are 10³–10¹¹ times less abundant than hydrogen.

The most accessible solar system materials are the rocks of the Earth's crust and upper mantle. Elemental abundance patterns in some terrestrial rocks do partially mimic those of the solar system as a whole. In general, however, igneous, metamorphic, and sedimentary processes operating over the 4.6 billion years of Earth history have strongly fractionated the chemical elements within the Earth. Determination of the overall chemical composition of the solar system requires information from other, less evolved members of the solar system. The principal data come from chemical analyses of meteorites, particularly the chemically primitive meteorites known as carbonaceous chondrites. Supplementary data are provided by atomic absorption spectra of the outer layer of the Sun.

The abundance patterns of the chemical elements record nucleosynthesis (production of the elements) in the Big Bang, in pre-solar system stars, and in the stellar supernova that triggered the formation of the solar system. The solar system abundance curve is dominated by two features: exponentially decreasing abundance with increasing atomic number, and sawtooth-like alternation of elements of even and odd atomic number. Heavier nuclei were produced by stepwise fusion of lighter nuclei and the addition of neutrons to lighter nuclei. Progressively heavier elements required more steps and therefore generally were produced less abundantly. Elements of even atomic number have more numerous stable isotopes than do odd-numbered nuclei, and are therefore more common. The iron-group elements (Mn, Fe, Co, Ni) are 10 to 1000 times more abundant than adjacent elements because they have the most stable nuclei, with the greatest binding energy per nuclear particle. The light metals lithium (Li), beryllium (Be), and boron (B) are grossly underabundant because they are consumed, rather than produced, in stars. Slightly elevated relative abundances of the heavy metals osmium (Os) through bismuth (Bi) are related to their nuclear stability and nucleosynthetic processes, as are other second-order features of the abundance curve.

Bibliography

Cox, P. A. (1989) The elements; their origin, abundance, and distribution. Oxford University Press.

G. B. Haxel