Oxygen

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Oxygen is a chemical element with the symbol O and atomic number 8, belonging to the chalcogen group on the periodic table. It is a highly reactive nonmetal and a powerful oxidizing agent that readily forms compounds, notably oxides, with most other elements. By mass, oxygen is the third-most abundant element in the universe after hydrogen and helium, and it is the most abundant element by mass in Earth's crust, forming almost half of that mass in the form of oxides such as silicates. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas that constitutes about 21% of Earth's atmosphere by volume. Continuously replenished in the atmosphere by photosynthesis, dioxygen is essential to most complex life on Earth due to its use in cellular respiration, which releases energy from organic compounds. Oxygen is also produced industrially, primarily through the fractional distillation of liquefied air, for uses ranging from steelmaking and medical therapy to rocket propellant, where liquid oxygen serves as an oxidizer in combination with fuels such as liquid hydrogen or kerosene.