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→Chemical Properties
Oxygen is a chemical [[element]] vital for life. Under normal conditions it is a colorless odorless [[gas]] consisting of two oxygen [[atom|atoms]] which has the chemical formula O<sub>2</sub>.
{{Element | name=Oxygen | symbol=O | anumber=8 | amass=15.994 amu | state=gas | class=Non-metal | cstructure=Cubic | color=Colorless | date=1774 | discname=[[Joseph Priestley]] | origname=From the Greek words ''oxus'' (acid) and ''gennan'' (generate) | uses=Supports life | obtained=From liquid air}}
'''Oxygen''' is the third most abundant element in the universe by mass and is essential to life on Earth.
==Chemical Properties==
At normal [[atmospheric pressure]] oxygen melts at 54.36 [[Kelvin]] (K) and boils at 90.2 K; on Earth it essentially exists only as a gas. It readily dissolves in water, which makes aquatic life possible. Under normal conditions oxygen forms molecules consisting of two oxygen atoms each sharing two electrons in the 2p subshell; this form of oxygen is known as O<sub>2</sub>. O<sub>2</sub> is a colorless, odourless gas. In liquid form (below -193 degrees Celsius) it is pale blue.
==Isotopes==
[[Category:Chemistry]]
There are three stable and 14 radioactive isotopes of oxygen. The stable isotopes are <sup>16</sup>O, <sup>17</sup>O and <sup>18</sup>O, with <sup>16</sup>O being by far the most common. Radioisotopes range from <sup>10</sup>O to <sup>26</sup>O, with the two most stable being <sup>15</sup>O and <sup>14</sup>O. This gives oxygen an atomic weight of slightly below 16.<ref>http://education.jlab.org/itselemental/iso008.html</ref>
[[Category:Elements]]
[[Category: hydrogen economy]]
==Compounds==
[[Category:gasses]]
Oxygen is highly reactive. It reacts readily with most metals and many other elements, either through combustion or oxidation. As a result, many naturally occurring compounds contain oxygen.
==Occurrence in Nature==
Oxygen is the third most abundant element in the universe by mass after [[hydrogen]] and [[helium]]; approximately 5.9% of the mass of our [[solar system]] consists of oxygen. However, due to the large disparity in atomic weight between all isotopes of oxygen compared with <sup>1</sup>H and <sup>4</sup>He, less than 0.5% of the atoms in the solar system are oxygen atoms.{{fact}}
Due to its reactivity free oxygen is extremely rare in nature and would not exist on Earth without the process of [[photosynthesis]]. Geologists and paleontologists believe that free oxygen first appeared during the "Great Oxygenation Event" around 2.4 billion years ago. Young-Earth creationists reject this timescale.
{{Periodic table}}
==References==
{{Reflist}}