Chemical Evolution
Charles Darwin introduced his theory of biological evolution in The Origin of Species, published in 1859. This development made people wonder what else in Nature had evolved; could, for example, chemical elements evolve? This question was answered in the mid-twentieth century.
In 1950s, several researchers from the United States concluded that all chemical elements originate from hydrogen. The notion that all elements derive from hydrogen was not a completely new hypothesis. In the nineteenth century, a couple of chemists proposed that chemical elements originated from hydrogen. The mechanism for the evolution of elements from hydrog en though was not developed until 1957 by Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle, who presented their mechanism in a paper entitled "Synthesis of the Elements in Stars." They proposed that the stars are the seat of origin of the elements, a process called nucleosynthesis. They concluded that they could expl ain the abundances of practically all the elements from hydrogen through synthesis in stars.
In 1812, Humphrey Davy (1778-1829) thought that hydrogen might be the primordial matter from which all other chemical elements derived, because he obtained hydrogen from most metals and from other unlikely materials. In 1815, William Prout (1785-1850) concluded that the atoms of all the chemical elements must have been formed by multiple combinations of hydrogen atoms. This conclusion was based on his finding that atomic weights were close to whole numbers. Prout calculated the specific gravities of elements and then compared the results with the specific gravity of hydrogen, which is one. All of his calculations produced whole numbers, and this led h im to wonder whether hydrogen was the basis of all matter, whether all the chemical elements somehow evolved from hydrogen. An example of Prout's thinking is if the atomic weight of oxygen is 16, then 16 volumes of hydrogen had condensed to form this element.
Over the rest of the nineteenth century developments were made in calculating the atomic weights of elements. Prout's hypothesis supposed that the atomic weights of elements should be whole numbers, but as the experiments and calculations improved, it was found that some elements did not have whole number atomic weights.Neon, for example, has an atomic weight of 20.18. In 1913, F. W. Aston (1877 1945) and J. J. Thomson (1856-1940) found that neon is composed of two main species, or isotopes, with atomic weights of 20 and 22 in the approximate ration of 10:1. Finding that the isotopes of neon have atomic weights that are whole numbers vindicated Prout's hypothesis that hydr ogen was the basis for all the elements. The evidence was building in favour of the hypothesis that hydrogen was the source for the evolution of elements.
Henry Russel (1877-1957) in 1929 demonstrated that hydrogen is the major elementary constituent of the Sun. This led to the hypothesis that the origin of the elements was in the stars, a process known as stellar nucleosynthesis. There were two mechanisms put forward to account for how the hydrogen in stars, like the Sun, could evolve into th e all of the elements we know of today. The first and earliest mechanism was based on a high- temperature equilibrium between atomic nuclei, giving the relative distribution of elements upon cooling. The second mechanism was a kinetic mechanism whereby the elements evolve sequentially with the addition of protons and neutrons from the h ydrogen atom.
The mechanism put forward by the Burbidges, Fowler, and Hoyle combined the two earlier proposed mechanisms. They proposed eight main nucleosynthetic processes in star interiors, all starting from hydrogen. The evolution of chemicals follows these eight processes: 1) the conversion of hydrogen to helium; 2) the combustion of helium to carbon, oxygen, and neon; 3) the capture of successive alpha particles by oxygen and neon producing magnesium, silicon, argon, and calcium; 4) an equilibrium process to allow for the high abundance of iron group elements; 5) a slow process of neutron capture by iron grou p elements (iron, cobalt, and nickel); 6) a rapid process of neutron capture in supernovae explosions; 7) a proton capture process producing the rare light isotopes of the heavy elements; and 8) a light-atom process where high- energy collisions break up heavier nuclei.
Davy and Prout then, early in the nineteenth century, were not wrong in thinking that hydrogen was the primary matter that all elements evolved from. Darwin's theory of biological evolution equipped scientists with a mechanism to show how all chemical elements originate from hydrogen.
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