Editing Period 2 element (section)

===Oxygen===
{{main article|Oxygen}}Oxygen is the chemical element with atomic number 8, occurring mostly as <sup>16</sup>O, but also <sup>17</sup>O and <sup>18</sup>O.

Oxygen is the third-most common element by mass in the universe (although there are more carbon atoms, each carbon atom is lighter). It is highly electronegative and non-metallic, usually diatomic, gas down to very low temperatures. Only fluorine is more reactive among non-metallic elements. It is two electrons short of a full octet and readily takes electrons from other elements. It reacts violently with [[alkali metals]] and [[white phosphorus]] at room temperature and less violently with alkali earth metals heavier than magnesium. At higher temperatures it burns most other metals and many non-metals (including hydrogen, carbon, and sulfur). Many oxides are extremely stable substances difficult to decompose—like [[water]], [[carbon dioxide]], [[alumina]], [[silica]], and iron oxides (the latter often appearing as [[rust]]). Oxygen is part of substances best described as some salts of metals and oxygen-containing acids (thus nitrates, sulfates, phosphates, silicates, and carbonates.

Oxygen is essential to all life. Plants and [[phytoplankton]] photosynthesize water and carbon dioxide and water, both oxides, in the presence of sunlight to form [[sugar]]s with the release of  oxygen. The sugars are then turned into such substances as cellulose and (with nitrogen and often sulfur) proteins and other essential substances of life. Animals especially but also fungi and bacteria ultimately depend upon photosynthesizing plants and phytoplankton for food and oxygen.

[[Fire]] uses oxygen to oxidize compounds typically of carbon and hydrogen to water and carbon dioxide (although other elements may be involved) whether in uncontrolled conflagrations that destroy buildings and forests or the controlled fire within engines or that supply electrical energy from turbines, heat for keeping buildings warm, or the motive force that drives vehicles.

Oxygen forms roughly 21% of the Earth's atmosphere; all of this oxygen is the result of photosynthesis. Pure oxygen has use in medical treatment of people who have respiratory difficulties. [[oxygen toxicity|Excess oxygen is toxic]].

Oxygen was originally associated with the formation of acids—until some acids were shown to not have oxygen in them. Oxygen is named for its formation of acids, especially with non-metals. Some oxides of some non-metals are extremely acidic, like [[sulfur trioxide]], which forms [[sulfuric acid]] on contact with water. Most oxides with metals are alkaline, some extremely so, like [[potash|potassium oxide]]. Some metallic oxides are amphoteric, like aluminum oxide, which means that they can react with both acids and bases.

Although oxygen is normally a diatomic gas, oxygen can form an allotrope known as [[ozone]]. Ozone is a triatomic gas even more reactive than oxygen. Unlike regular diatomic oxygen, ozone is a toxic material generally considered a pollutant. In the upper atmosphere, some oxygen forms ozone which has the property of absorbing dangerous ultraviolet rays within the [[ozone layer]]. Land life was impossible before the formation of an ozone layer.