Oxygen

Oxygen, a chemical element with symbol O and atomic number 8, is a highly reactive nonmetal and oxidizing agent. It readily forms oxides with most elements and compounds. Oxygen is the most abundant element in Earth's crust and the third-most abundant in the universe after hydrogen and helium. Two oxygen atoms bond to form dioxygen (O₂), a colorless, odorless diatomic gas that makes up 20.95% of Earth's atmosphere. Oxygen also constitutes nearly half of the Earth's crust as oxides.