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'''Oxygen''' is a [[chemical element]] with the chemical symbol '''O''' and [[atomic number]] 8. On Earth, it is usually [[Covalent bond|covalently]] or   [[ionic bond|ionically]] bonded to other elements. 

Unbound oxygen (also called molecular oxygen, or dioxygen, O<sub>2</sub>, 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 [[anaerobe]]s ([[archaea]] and [[bacteria]]). The presence of large amounts of free oxygen may have driven most of the organisms then living to extinction.{{citation needed}} 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 of the free element being produced by [[algae]] in the oceans, and one quarter from terrestrial [[plant]]s.

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