In science, oxygen (IPA: /ˈɒksɪdʒən/) is a chemical element with the chemical symbol O and atomic number 8. The word oxygen derives from two roots in Greek, οξύς (oxys) (acid, lit. sharp) and -γενής (-genēs) (producer, lit. begetter). In the early 18th century, Antoine Lavoisier coined the name oxygen from the Greek roots mentioned above because he erroneously thought that it was a constituent of all acids. (The definition of acid has since been revised). Oxygen has a valency of 2. On Earth it is usually bonded to other elements covalently or ionically. Examples for common oxygen-containing compounds include water (H2O), sand (silica, SiO2), and rust (iron oxide, Fe2O3).
Diatomic oxygen (O2) is one of the two major components of air (20.95%). It is produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. It is toxic to obligate anaerobic organisms and was a poisonous waste product for early life on Earth.
Triatomic oxygen (ozone, O3) forms through radiation in the upper layers of the atmosphere and acts as a shield against UV radiation.