Oxygen

Oxygen (pronounced /??ks?d??n/) is a colorless, odorless, tasteless, gaseous chemical element with the chemical symbol O and atomic number 8. It is a chalcogen, period 2, nonmetallic element that can form binary compounds (known as oxides) with almost all the other elements. The valency of oxygen is 2 and the most common oxidation state is -2. On Earth it is usually bonded to other elements covalently or ionically. Oxygen is the third most abundant element in the universe by mass (after hydrogen and helium), the most abundant element by mass in the Earth’s crust, as well as the most abundant element by mass in the human body.

The word oxygen derives from two roots in Greek, (acid, lit. “sharp,” from the taste of acids) and (producer, lit. begetter). It was recognized in 1777 by Antoine Lavoisier, who coined the name oxygen from the Greek roots mentioned above because he erroneously thought that it was a constituent of all acids.^[5]

Diatomic oxygen or dioxygen (O₂) is, together with nitrogen, one of the two major components of air, constituting about a fifth of its volume. Dioxygen is produced from water by plants during photosynthesis, and is necessary for aerobic respiration in animals. Without oxygen, most organisms with aerobic respiration will die within minutes. However, it is toxic to obligate anaerobic organisms and was a poisonous waste product for early life on Earth. Triatomic oxygen (ozone, O₃) is formed by reaction of O₂ with atomic oxygen produced by UV radiation of O₂ in the upper atmosphere. Ozone absorbs strongly in the ultraviolet and acts as a shield for the biosphere against the mutagenic and other damaging effects of UV radiation.

Arguably, the most familiar oxygen compound is water, the oxide of hydrogen H₂O. However the earth’s crustal rock is predominantly composed of oxides of silicon as silica, SiO₂ (found in granite and sand), silicates found in feldspars and glass, and oxygen compounds of metals such as calcium (as calcium carbonate in limestone), aluminium (as silicates in feldspars and as aluminium oxide in bauxite and corundum), iron (as iron (III) oxide Fe₂O₃ in hematite and rust), etc. Other well-known examples of oxygen compounds include the compounds of carbon and oxygen, such as carbon dioxide (CO₂), alcohols (R-OH where “R” is an organic group), carbonyls (R-CO-H or R-CO-R) such as acetone; and carboxylic acids (R-COOH) such as fatty acids. Oxygenated radicals such as perchlorates (ClO₄^?) and nitrates (NO₃^?) are strong oxidizing agents in and of themselves. Phosphorus is biologically important in its oxygenated form as the phosphate (PO₄^3?) ion and as the backbone of RNA and DNA.