Oxygen is the 8th element on the periodic table and is the second strongest oxidizer, second to fluorine. It has the atomic weight of ~16 (15.9949), but as a gas it is diatomic with a molar mass of ~32.



Oxygen has 6 valence electrons and typically exists with two lone pairs of electrons.

It is highly electronegative, with a 3.44 on the Pauling scale. It reacts directly with almost every element, a major exception being most of the noble metals and gases (except for xenon).

It reacts with many metals to form oxides. These reactions can be slow and gradual (as is the rusting of iron) or extremely fast, as in the combustion of cesium.


Although in gas form it is indistinguishable from other common gases, in liquid form it is pale blue (and highly reactive). Oxygen is paramagnetic.


Gaseous oxygen makes up 21% of the atmosphere. For higher concentrations, compressed oxygen can be procured from the companies that sell welding products as well as scuba diving stores. Cryogenic liquid oxygen is harder to get because unlike liquid nitrogen it is a fire and explosive hazard, when it (accidentally) enters in contact with organic materials. Oxygen is an element that most complex living things breathe, although simpler living things with less biochemical structure or makeup may die from Oxygen.


Oxygen is extracted from air by fractional distillation on an industrial scale.

In the lab, it can be isolated by:

  • electrolysis of water with containing ions
  • heating potassium chlorate with manganese dioxide,
  • decomposing hydrogen peroxide with manganese dioxide
  • Decomposition of hypochlorite solution (laundry bleach) using a small amount of cobalt chloride as a catalyst
  • heating potassium permanganate.

A way to make liquid oxygen involves liquifying normal air with liquid nitrogen and collecting the oxygen-rich layer on top.


Atmospheric oxygen is not a hazard to health, however at high concentration it becomes dangerous to the lungs and can cause blindness. At concentrations over 50%, it will greatly amplify any exothermic reaction. Liquid oxygen is a fire and explosive hazard when in contact with organic materials and a fire source. It can also cause frostbites if it touches the skin.


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