Chemical Reactions

History of Chemical Reactions

The world as we know it, indeed the entire universe, is composed of the chemical elements. The 94 known chemical elements (there are more in the periodic table, but they do not occur naturally) were formed shortly after the start of time as we know it—the big bang—and have been combining with each other ever since. Shortly after the big bang, extreme temperatures are believed to have favored the fusion of the separate neutrons and protons present, forming helium and deuterium nuclei, in a process termed big bang nucleosynthesis. These positively charged nuclei subsequently combined with the negatively charged primordial particles present, electrons, to produce the first atoms, of which most were hydrogen. The extreme conditions generated by the production of the first stars are believed to have created heavier elements through the processes of stellar and supernova nucleosynthesis, and cosmic ray spoliation. The first chemical reactions would have now occurred, with simple chemical entities based on light atoms such as carbon, nitrogen, and oxygen produced in abundance.

The formation of the earth (c. 4.6 billion years ago) was followed by the Hadean eon, where the earth began to take shape. Changes in the chemical composition of the atmosphere, afforded by volcanic eruptions, the cooling of the earth's surface, and asteroid bombardment, have been proposed to have been instrumental in the development of life. The generation of small organic molecules, known to be monomeric constituents of living organisms from the most basic chemical species, is propounded as one of the most widely accepted theories of the origin of life. The combination of a-amino acid molecules to form polypeptide chains; the similar production of nucleotides, phosphates, and sugars could, theoretically, react together in such a way as to give the double-helix of life, DNA. The classic Miller-Urey experiment exposed a mixture of simple gases and water vapor to an electrical discharge (to mimic lightning), and the ensuing chemical reactions were indeed found to produce simple biomolecules that are essential to life. An early earth atmosphere rich in ammonia, methane, water, carbon dioxide, and nitrogen could have been transformed chemically to give a so-called primordial soup.

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