Oxygen
Oxygen is the simplest group VIA element and is, under normal atmospheric conditions, usually found as a colorless, odorless, and tasteless gas. Oxygen has an atomic number of 8 and an atomic mass of 16.0 amu. The liquid and solid forms, which are strongly paramagnetic, are a pale blue color. Oxygen has a boiling point of −297°F (−182.8°C) and a melting point of −368.7°F (−222.6°C).
Oxygen is the third most abundant element found in the Sun, after hydrogen and helium, and plays an important role in the carbon-nitrogen cycle. Oxygen composes 21% of Earth's atmosphere by volume and is vital to the existence of carbon-based life forms.
Although English chemist Joseph Priestley (1733–1804) is generally credited with the discovery of oxygen in 1774, many science historians contend that Swedish chemist Carl Scheele (1742–1786) probably discovered oxygen a few years prior to Priestly. French chemist Antoine Lavoisier's (1743–1794) contributions to the study of the important reactions, combustion and oxidation, were spurred by the discovery of oxygen. Lavoisier noticed that something was absorbed when combustion took place and that it was obtained from the surrounding air. Lavoisier noted that the increase in the weight of the substance burned was equal to the decrease in the weight of the air used. His studies lead to Lavoisier's oxidation theory, which eventually superseded the phlogistonists' theory (i.e., that every combustible substance was thought to contain a phlogiston, or inherent principal of fire, liberated through burning, along with a residue) that was widely accepted at that time. Lavoisier eventually named the gas he studied oxygen from the Greek oxys meaning acid or sharp, and geinomial meaning forming. Lavoisier named the gas oxygen because he noted that the burned materials were converted into acids.
Although oxygen has nine isotopes, natural oxygen is a mixture of only three of these. The most abundant isotope, oxygen-18, is stable and available commercially. The most common use for commercial oxygen gas is in enrichment of steel blast furnaces and for medical purposes. Large quantities are also used in making synthetic ammonia gas, methanol and ethylene oxide. Oxygen is also consumed in oxy-acetylene welding. Most commercial oxygen is produced in air separation plants. It is estimated that the United States consumes 20 million tons of oxygen in commercial use per year and the demand is expected to increase dramatically.
When oxygen is exposed to ultraviolet light, as from the Sun, or an electrical discharge, as from lightening, ozone (O3) is formed. Although ozone is toxic to breathe, the 0.12 in (3 mm) thick layer of ozone in the earth's atmosphere provides a shield from harmful ultraviolet rays from the Sun. The ozone layer has recently been the subject of intense scientific interest to determine whether, and to what extent, it may be deteriorating, mainly from pollutants in the atmosphere. Unlike pure oxygen gas, ozone has a bluish color and its liquid and solid forms are bluish black to violet-black.
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