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Element Fact
Selenium is an essential trace element in the diets that acts as antioxidants.

Oxygen, O

oxygen sample

Atomic number: 8
Atomic mass: 15.9994(3)
Natural abundance: 474000 ppm
16O (15.994915) - 99.762%
17O (16.999132) - 0.038%
18O (17.999160) - 0.200%

History and Production

In Greek, oxys means sharp, acid and genes means forming. It was discovered in 1773-4 by J. Priestley and independently by C.W. Scheele. However, there had been earlier reports oxygen being prepared but did not recognized as an element. Scheele, a Swedish pharmacist, called the gas vitriol air and found to support combustion better than air. However, it was A. L. Lavoisier that recognized the gas as an element and named the element oxygen in 1777. The recognition of the oxgen as a combustible substance led to the demise of the phlogiston theory and laid the foundations of modern chemistry.

Nowadays, it can be isolated from Earth atmosphere using liquefaction and distillation techniques. Laboratory preparations can be achieved by heatng KClO3 to about 450C. Oxygen is used on a large scale production of titanium dioxide, ethylene oxide, vinyl chloride etc. It is also used for environmental purposes such as sewage treatment, river revival and fish farming. Large scale liquid oxygen is used as an oxidant in rocket fuels in space science and spacecraft launching.

Physical Data

Oxygen is the most abundant element on the Earth's surface. It is also the third most abundant element found in the Sun. The earth atmosphere consists of 21% of oxygen by volume and its compounds make up to about 49%, by weight, of the earth's crust. The gas is colorless, odorless, tasteless but is highly reactive. Liquid and solid oxygen are pale blue in color and are strongly paramagnetic. The triatomic oxygen, ozone, has a bluish color in appearance. The liquid form is bluish black, while the solid form is violet-black. Ozone can be detected by its pungent odour in concentrations down to 0.01 ppm.

Interatomic distance: 120.8 pm
Melting point: -218.75C
Boiling point: -182.96C
Thermal conductivity/Wm-1K-1: 0.2674 (27C)
Density/kgm-3: 2000 (m.p.), 1140 (b.p.), 1.429 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 249.2 kJ/mol
Gibbs free energy of formation: 231.7 kJ/mol
Entropy: 161.1 J/mol K
Heat capacity: 21.9 J/mol K

Electronic data

Electronic configuration: [He] 2s2 2p4
Term symbol: 3P2
Electron affinity: 140.9757 kJ/mol   Electronegativity (Pauline): 3.44
Ionization energy (first, second, third): 1313.94, 3391.818, 5300.474 kJ/mol

Chemical Properties

Oxygen is a high reactive elements that combines directly with most metal and non-metal elements to form oxides. The few exceptions are the noble gases and precious metals such as platinum and gold. Liquid oxygen can produce spectacular combustion effects. For instance, a cotton wool soaked in liquid oxygen explodes when ignited.

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