# Oxygen

Oxygen is a group 16 element and is very reactive, reacting with metals and nonmetals to form various compounds.

Oxygen is an oxidizing agent, it receives electrons from atoms that it reacts with. Oxygen has six valence electrons and commonly has a –2 oxidation state. It typically forms two bonds, either two single bonds or one double bond. There are several known allotropes of oxygen, different physical forms in which oxygen can exist. The most common is O2, which is most prevalent in Earth's atmosphere.

Oxygen and nitrogen are the most abundant elements found in air. Commercial oxygen is obtained from air. Pure oxygen is widely used in many industries due to its strong oxidizing property.

An oxide is a compound consisting of oxygen and one other element. Oxygen forms oxides with both metals and nonmetals. Metal oxides are often ionic compounds. Nonmetal oxides are often covalent molecules. Oxides can form acids or bases when they react with water. An oxide that forms an acid in water is called an acid anhydride, meaning "acid without water." An oxide that forms a base in water is called a base anhydride, meaning "base without water." Generally, nonmetal oxides are acid anhydrides, and metal oxides are base anhydrides. However, exceptions exist in both cases.
A peroxide is a compound that has an oxygen-oxygen bond in which the oxidation state of oxygen is –1 instead of the usual –2. Hydrogen peroxide (H2O2), the most common peroxide, is a thick liquid that decomposes into water and oxygen in a strongly exothermic reaction:
${2\rm{H_2O_2}}(l)\rightarrow{2}{\rm{H_2O}}(l)+{\rm{O_2}}(g)$
Note that in this reaction the oxygen in hydrogen peroxide (H2O2) gets both reduced and oxidized. The oxygen has an oxidation state of –1 in peroxide. In water (H2O), the oxidation state is –2. In oxygen gas (O2), the oxidation state is 0. A reaction in which an element is both reduced and oxidized is called a disproportionation reaction. Hydroxide is an ion that has one oxygen and one hydrogen atom. The formula of a hydroxide ion is OH. In pure water some of the water molecules break into hydronium (H3O+) and hydroxide ions:
${2\rm{H_2O}}(l)\rightleftharpoons{\rm{H_3O}^+}(aq)+{\rm{OH^-}}(aq)$
Hydroxide ions form compounds with metals to form hydroxides. Two common hydroxides are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Hydroxides release hydroxide ions in water, making them basic. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are both strongly basic.
A carbonate is a substance with the carbonate ion (CO32–). A related substance is a hydrogen carbonate, which is a substance with the HCO3 ion. Compounds of carbonate and hydrogen carbonate are common in nature. For example, calcium carbonate (CaCO3) is called calcite and is the principal component of limestone. Shells of many animals are made up of calcite. At high temperatures, calcium carbonate (CaCO3) decomposes into calcium oxide (CaO) and carbon dioxide (CO2). Calcium oxide (CaO) is called quicklime and is the main component of cement and concrete. A commonly used bicarbonate is sodium bicarbonate (NaHCO3), also called baking soda. Bicarbonates also play an important role regulating acidity in the body.