Oxygen is a chemical element. In the periodic table it has the symbol O and atomic number 8. Oxygen is the second most common element on Earth composing around 49% of the mass of Earth's crust^* and 28% of the mass of Earth as a whole, and is the third most common element in the universe. On Earth, it is usually covalently or ionically bonded to other elements. Unbound oxygen (usually called molecular dioxygen, O₂, a diatomic molecule) first appeared in significant quantities on Earth during the Paleoproterozoic era (between 2.5 billion years ago and 1.6 billion years ago) as a product of the metabolic action of early anaerobes (archaea and bacteria). This new presence of large amounts of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Characteristics

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At standard temperature and pressure, oxygen exists as a diatomic molecule with the formula O₂, in which the two oxygen atoms are doubly bonded to each other. In its most stable form, oxygen exists as a diradical (triplet oxygen). Though radicals are commonly associated with highly reactive compounds, triplet oxygen is surprisingly (and fortunately) unreactive towards most compounds. Singlet oxygen, a name given to several higher energy species in which all the electron spins are paired, is much more reactive towards common organic molecules. Carotenoids effectively absorb energy from singlet oxygen and convert it back into the unexcited ground state.

Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure. Hydrochloric acid (HCl) does not contain oxygen. Liquid O₂ and solid O₂ have a light blue color and both are highly paramagnetic. Liquid O₂ is usually obtained by the fractional distillation of liquid air. Liquid and solid O₃ (ozone) have a deeper color of blue.

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thinks.... friends are like oxygen - you can't live without them :): http://chatter.com/a/x9ql
smithy3370 (stephen smith) Sun, 03 Jan 2010 12:22:38 -0000
thinks.... friends are like oxygen - you can't live without them :): http://chatter.com/a/x9ql
RT @vcharasa:Miss u already boy,hiks :'( can't live without u near me. Oxygen feel diffrent when you're not here I can't breathe normally :(
farahjasmine (Farah Jasmine) Sun, 03 Jan 2010 12:22:01 -0000
RT @vcharasa:Miss u already boy,hiks :'( can't live without u near me. Oxygen feel diffrent when you're not here I can't breathe normally :(
Miss u already boy,hiks :'( can't live without u near me. Oxygen feel diffrent when you're not here I can't breathe normally : (
vcharasa (Vannya C Annissa) Sun, 03 Jan 2010 12:20:38 -0000
Miss u already boy,hiks :'( can't live without u near me. Oxygen feel diffrent when you're not here I can't breathe normally : (
RT @moko_oxygen 【速報】ばーちゃんちに iPhone の充電ケーブル忘れて来た
hb_86 (hb_86) Sun, 03 Jan 2010 12:18:52 -0000
RT @moko_oxygen 【速報】ばーちゃんちに iPhone の充電ケーブル忘れて来た
@moko_oxygen トラのキャラクターではないと。ジャンガリアンにも縞といえば縞かな、な一本線はありますねw
kmj_t (上條建) Sun, 03 Jan 2010 12:18:30 -0000
@moko_oxygen トラのキャラクターではないと。ジャンガリアンにも縞といえば縞かな、な一本線はありますねw
@moko_oxygen えっ…しまじろう…しましまとらの、とかいう？
kmj_t (上條建) Sun, 03 Jan 2010 12:13:30 -0000
@moko_oxygen えっ…しまじろう…しましまとらの、とかいう？

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