What is Oxygen?
{{Infobox oxygen}} '''Oxygen''' is a [[chemical element]]. It has the symbol '''O''', on the [[periodic table]] and [[atomic number]] 8. It is the third most common [[chemical element|element]] in the [[universe]], after [[hydrogen]] and [[helium]]. Oxygen is [[#Occurrence|more than a fifth]] of the [[Earth's atmosphere]] by volume. In the [[air]], two oxygen [[atom]]s usually [[chemical bond|join]] to make dioxygen ({{chem|O|2}}), a [[colour]]less [[gas]]. This gas is often just called oxygen. It has no [[taste]] or [[olfaction|smell]]. It is pale [[blue]] when it is [[liquid]] or [[solid]]. Oxygen is part of the [[Group 16 element|chalcogen]] [[group (periodic table)|group]] on the [[periodic table]]. It is a very [[chemical reaction|reactive]] [[nonmetal]]. It makes [[oxide]]s and other [[Chemical compound|compounds]] with many elements. The oxygen in these oxides and in other compounds (mostly [[silicate]] [[mineral]]s, and [[calcium carbonate]] in [[limestone]]) makes up [[#Occurrence|nearly half]] of the [[Earth's crust]], by [[mass]]. Most living things use oxygen in [[respiration]]. Many [[organic compounds|molecule]]s in living things have oxygen in them, such as [[protein]]s, [[nucleic acid]]s, [[carbohydrate]]s and [[fat]]s. Oxygen is a part of [[water]], which all known life needs to live. [[Algae]], [[cyanobacteria]] and [[plant]]s make the Earth's oxygen gas by [[photosynthesis]]. They use the [[Sun]]'s light to get hydrogen from water, giving off oxygen. At the top of the Earth's atmosphere is [[ozone]] ({{chem|O|3}}), in the [[ozone layer]]. It absorbs [[ultraviolet radiation]], which means less radiation reaches ground level. Oxygen gas is used for making [[steel]], [[plastic]]s and [[textile]]s. It also has medical uses and is used for breathing when there is no good air (by [[underwater diving|diver]]s and [[firefighter]]s, for example), and for [[welding]]. Liquid oxygen and oxygen-rich compounds can be used as a [[rocket propellant]]. ==History== Oxygen gas ({{chem|O|2}}) was [[chem