The chemical formula for oxygen gas is O₂. This means that one molecule of oxygen gas consists of two oxygen atoms bonded together. Oxygen is a diatomic molecule and is vital for most forms of life on Earth. It is essential for cellular respiration, a process through which cells produce energy by reacting oxygen with glucose. In its gaseous state at room temperature, oxygen is colorless and odorless. It constitutes approximately 21% of the Earth's atmosphere and is obtained from the air or produced through photosynthesis by plants.