Under [[normal condition]]s it is a colorless, odorless [[gas]] consisting of two oxygen [[atom|atoms]] which has the chemical formula O<sub>2</sub>. This means it is [[diatom]]ic. Approximately 20% of the [[atmosphere]] is made up of oxygen. Oxygen also forms a [[triatomic]] molecule called [[ozone]] (O<sub>3</sub>), which although unstable under normal conditions, is a very important ingredient in the upper [[atmosphere]]. | Under [[normal condition]]s it is a colorless, odorless [[gas]] consisting of two oxygen [[atom|atoms]] which has the chemical formula O<sub>2</sub>. This means it is [[diatom]]ic. Approximately 20% of the [[atmosphere]] is made up of oxygen. Oxygen also forms a [[triatomic]] molecule called [[ozone]] (O<sub>3</sub>), which although unstable under normal conditions, is a very important ingredient in the upper [[atmosphere]]. |