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Symbol O. The element with atomic number 8 and relative atomic mass 16.00. It is the element in Group 16 with the lowest atomic number, and is a nonmetal. It is allotropic: one allotrope is ozone; the much more common one is dioxygen, O{TAG(tag=>sub)}2{TAG}, which is a colourless odourless gas.

Oxygen is a very reactive element, reacting often violently with metals such as magnesium, nonmetals such as sulphur, inorganic compounds such as sulphur dioxide and organic compounds such as methane. Burning in oxygen is called combustion, and the majority of mankind\'s energy needs are met by combustion. Liquid oxygen, which is coloured blue, is particularly violent in its reactions. Combined with a fuel of liquid hydrogen it is used to power the space shuttle.

Oxygen is unusual in being a diradical (i.e. having two unpaired electrons), and the Lewis structure cannot account for this observation. An explanation is provided by molecular orbital theory, which shows that the two unpaired electrons arise because two degenerate orbitals have to be occupied (see the diagram) .

Oxygen is essential for life; thus the first question a first-aider must answer is ‘Is the casualty breathing?’ Humans can do without food for weeks in extremis, water for a few days, but oxygen for only a matter of minutes.See oxygen transport.

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