* Oxygen is a member of the main group nonmetals family:
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OXYGEN / O / 8
Elemental oxygen occurs as O2, a colorless, odorless, moderately
reactive gas. Three isotopes are found in nature:
O<16/8> / 99.76%
O<18/8> / 0.2%
O<17/2> / 0.04%.
All are stable.
atomic weight: 15.994
abundance: 1st
density of O2: 1.429 gm/l
melting point: -218 C
boiling point: -183 C
valence: 2
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The "O--" ion is of course known as "oxide", while the "O2--" ion is known as "peroxide". The "OH-" group is known as "hydroxide".
Oxygen gas makes up about 21% of the Earth's atmosphere by volume. It readily reacts with a wide range of other elements, creating "oxide" compounds, and in fact if it were not continuously replenished by plant respiration, it would all combine with carbon and disappear from the atmosphere. Any distant planet with an appreciable amount of O2 in its atmosphere is likely to support life. Much of the Earth's crust is made of oxides, particularly silicon dioxide (SiO2).
High-energy radiation from the Sun and space tends to ionize oxygen in the upper atmosphere, which leads to the creation of the ozone (O3) molecule. Ozone is colorless but highly reactive and has a definite odor, its smell being obvious in thunderstorms. Ozone is dangerous to breathe in any real quantity and is a significant air pollutant, but since it absorbs high-energy radiation, the ozone layer in the upper atmosphere is important to the preservation of life on Earth.
As with nitrogen, oxygen is obtained industrially from the air by fractional distillation, with about a hundred million tonnes extracted each year. It is used to supply breathing apparatus, oxy-acetylene welding, and a range of industrial processes, including smelting of iron and production of steel.
