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Introduction

Smithsonian Institution, PO Box 37012, MRC 119, Washington, D.C. 20013-7012, U.S.A., macphers@si.edu

The first 20% of the full text of this article appears below.

Oxygen (O) Atomic No. = 8 Atomic Wt. = 15.9994 Solar System Abundance (relative to 106 Si atoms)1 = 1.413 x 107 Three stable isotopes: 16O, 17O, 18O Relative isotopic abundances on Earth2: 99.762% 16O, 0.038% 17O, 0.200% 18O First isolated as an element in the late 18th century by Scheele and Priestley (independently), oxygen is the third most abundant element in the universe. It is abundant on Earth as a colorless elemental gas in the atmosphere, in combination with hydrogen in water, and in combination with silicon and other metals in the silicates and oxides that make up Earth’s crust and mantle. Oxygen is chemically very electronegative, and only a few metals (mainly, the platinum group metals, gold, silver, mercury, and copper) persist in their elemental form on Earth’s surface in the presence of the oxygenated atmosphere. The abundance of elemental oxygen in Earth’s atmosphere is more or less steady-state, being produced and sustained by the action of photosynthetic plants against the constant removal by physical and biogenic oxidation processes.

 

Hydrogen may be the most abundant element in the universe, but in science and in nature oxygen has an importance that is disproportionate to its abundance. Human beings tend to take it for granted because it is all around us and we breathe it, but consider the fact that oxygen is so reactive that in a planetary setting it is largely unstable in its elemental state. Were it not for the constant activity of photosynthetic plants and a minor amount of photo dissociation in the upper atmosphere, we would not have an oxygen-bearing atmosphere and we would not be here. Equally, the most important compound of oxygen is water, without which life (in the sense that we know it) could not exist. . . . [Full Text of this Article]