The element with the chemical symbol O is oxygen. Oxygen is a colorless, odorless gas essential for life on Earth, making up approximately 21% of the atmosphere. It is a highly reactive non-metal that readily forms compounds with most elements and plays a crucial role in processes such as combustion, respiration, and photosynthesis. In the biosphere, oxygen is vital for the survival of aerobic organisms, including humans, who depend on it to produce energy through cellular respiration. In the periodic table, oxygen has an atomic number of 8 and is classified as a member of the chalcogen group.