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There are three allotropes (different forms) of oxygen: (1) atomic oxygen (O), sometimesreferred to as nascent or “newborn” oxygen; (2) diatomic oxygen (O2), or molecular oxygen(gas); and (3) ozone (O3), also a gas.The atmospheric oxygen that we breathe is a very reactive nonmetal and is colorless, odorless,and tasteless, but it is essential to all living organisms. It readily forms compounds withmost other elements. With six electrons in its outer valence shell, it easily gains two moreelectrons to form a negative (–2) ion; or as covalent, it can share electrons with other elementsto complete its outer shell.Almost all the oxygen in the atmosphere (?21%) is the allotropic form of molecular oxygen(O2). This essential gas we breathe is the result of photosynthesis, which is how green plants(with chlorophyll) use the energy of the sun to convert carbon dioxide (CO2) and water tostarches and sugars with molecular oxygen as the by-product.Liquid oxygen has a slightly bluish cast to it. As it boils, pure oxygen gas is released. Themelting point for oxygen is –218.79°C, its boiling point is –182.95°C, and its density is0.001429 g/cm3.