Oxygen and its compounds play a key role in many of the important processes of life and industry. Oxygen in the biosphere is essential in the processes of respiration and metabolism, the means by which animals derive the energy needed to sustain life. Furthermore, oxygen is the most abundant element at the surface of the Earth. In combined form it is found in ores, earths, rocks, and gemstones, as well as in all living organisms. Oxygen is a gaseous chemical element in Group VA of the periodic table. The chemical symbol for atomic oxygen is O, its atomic number is 8, and its atomic weight is 15.9994. Elemental oxygen is known principally in the gaseous form as the diatomic molecule, which makes up 20.95% of the volume of dry air. Diatomic oxygen is colorless, odorless, and tasteless. Two 18th-century scientists share the credit for first isolating elemental oxygen: Joseph PRIESTLEY (1733-1804), an English clergyman who was employed as a literary companion to Lord Shelburne at the time of his most significant experimental work, and Carl Wilhelm SCHEELE (1742-86), a Swedish pharmacist and chemist. It is generally believed that Scheele was the first to isolate oxygen, but that Priestley, who independently achieved the isolation of oxygen somewhat later, was the first to publicly announce his findings. The interpretation of the findings of Priestley and the resultant clarification of the nature of oxygen as an element was accomplished by the French scientist Antoine-Laurent LAVOISIER (1743-94). Lavoisier's experimental work, which extended and improved upon Priestley's experiments, was principally responsible for the understanding of COMBUSTION and the establishment of the law of conservation of matter. Lavoisier gave oxygen its name, which is derived from two Greek words that mean "acid former." Lavoisier held the mistaken idea that oxides, when dissolved in water, would form only acids. It is true that some oxides when dissolved in water do form acids; for example, sulfur dioxide forms sulfurous acid. Some oxides, however, such as sodium oxide, dissolve in water to form bases, as in the reaction to form sodium hydroxide; therefore oxygen was actually inappropriately named.
Oxygen is formed by a number of nuclear processes that are believed to occur in stellar interiors. The most abundant isotope of oxygen, with mass 16, is thought to be formed in hydrogen-burning stars by the capture of a proton by the isotopes of nitrogen and fluorine, with the subsequent emission of, respectively, a gamma ray and an alpha particle. In helium-burning stars the isotope of carbon with mass 12 is thought to capture an alpha particle to form the isotope with mass 16 with the emission of a gamma ray. In the terrestrial environment oxygen accounts for about half of the mass of the Earth's crust, 89% of the mass of the oceans, and 23% of the mass (and 21% of the volume) of the atmosphere. Most of the Earth's rocks and soils are principally silicates. The silicates are an amazingly complex group of materials that typically consist of greater than 50 (atomic) percent oxygen in combination with silicon and one or more metallic elements. Several important ores are principally oxides of the desired metals, such as the important iron-bearing minerals hematite, magnetite, and limonite and the most important aluminum-bearing mineral, BAUXITE (a mixture of hydrated aluminum oxides and iron oxide).
PHYSICAL AND CHEMICAL PROPERTIES
Three naturally occurring isotopes of oxygen have been found: one with mass 16 (99. 759% of all oxygen), one with mass 17 (0.037%); and one with mass 18 (0.204%). The rarer isotopes, principally the latter, find their major use in labeling experiments used by scientists to follow the steps of chemical reactions. If oxygen at a pressure of one atmosphere is cooled, it will liquefy at 90.18 K (-182.97 deg C; -297.35 deg F), the normal boiling point of oxygen, and it will solidify at 54.39 K...