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In science, oxygen (IPA: ) 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 (H<sub>2</sub>O), sand (silica, SiO<sub>2</sub>), and rust (iron oxide, Fe<sub>2</sub>O<sub>3</sub>). Oxygen is critical for life on earth. Diatomic oxygen (O<sub>2</sub>) is one of the two major components of air. It is produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. Triatomic oxygen (ozone, O<sub>3</sub>) forms through radiation in the upper layers of the atmosphere and acts as a shield against UV radiation.