OXYGEN

A colourless, odourless, two-atomgas (O2), the third most abundant element in the universe (after hydrogen and helium), the most abundant in the Earth’s crust (comprising almost 50 per cent by weight), and the second most abundant in the atmosphere (after nitrogen). The element is formed as part of the fuel cycle that provides the energy of stars. The Earth’s crust is mainly silicate minerals (minerals of silicon (Si) and oxygen). Earth is the only planet with an oxygen-rich atmosphere, which was produced by plants as a by-product of photosynthesis. Oxygen is essential for all forms of life as it is a constituent of dna, carbon dioxide and water, and it is required by all plants and animals for respiration. Oxygen is a highly reactive gas, permitting combustion and forming oxides with almost all other elements. It is also used widely in industry where most commercially produced oxygen is used in the manufacture of steel (>50 per cent) and chemicals (~25 per cent).

[See alsoatmospheric composition, biochemical oxygen demand (bod), earth revolutions, oxygen cycle, oxygen isotopes, oxygen variations, ozone, silica]