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A chemical element with the symbol O and atomic number 8, oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms oxides with most elements as well as with other compounds. It is a key component of Earth's atmosphere, making up about 21% of its volume and playing a critical role in respiration and combustion processes. Oxygen is essential for most forms of life, as it is used in cellular respiration to generate energy from food. It is also a major component of water and many minerals, contributing to the formation of rocks and soils. In its diatomic form (O2), it is the most abundant element by mass in the Earth's crust and is crucial for sustaining life on the planet.