Oxygen (abbreviated as O) is the third most common element in the universe, after hydrogen and helium. As a molecule, oxygen usually binds to make dioxygen (O2), a colorless gas without taste or smell that makes up 20.8% of the Earth's atmosphere. Ozone (O3) is another oxygen molecule and is an important greenhouse gas that absorbs ultraviolet radiation to reduce the amount of radiation that reaches the Earth's surface. Oxygen is very reactive and makes oxides, such as bonding with iron to make rust. Oxides make up nearly half of the Earth's crust.
All known life require water, which oxygen is a component of, and during photosynthesis, plants utilize sunlight to separate oxygen from water and carbon dioxide. The separated oxygen is used by most life for aerobic respiration.