Oxygen (O) is a colorless, odorless, gaseous element, the most abundant in Earth's crust (49.2% by mass) and the second most abundant in the atmosphere (28% by volume). Free oxygen is essential to the survival of all aerobic organisms.
Oxygen was discovered by Joseph Priestley in Leeds in 1774, and independently, and perhaps a year earlier, by Carl Scheele in Uppsala, Sweden. It was named by Antoine Lavoisier from the Greek oxy genes meaning "acid forming".
Chemistry of oxygen
|relative atomic mass||15.9994|
|atomic radius||66 pm|
|boiling point||-183.0 ºC|
Allotropes of oxygen
Isotopes of oxygen
Oxygen has three stable isotopes: 16O (99.757%), 17O (0.038%), and 18O (0.205%). Variations in the 18O/16O and 17O/16O ratios (for example, in meteorites) are reported in terms of their deviations (δ18O and δ17O) in parts per thousand from a standard. The usual reference is standard mean ocean water (SMOW).
Industrial production and laboratory preparation of oxygen
One way to prepare oxygen in the lab is to heat potassium chlorate crystals (KClO2) with which black manganese dioxide powder has been mixed. The potassium dichlorate decomposes – oxygen is set free and potassium chloride (KCl) is left behind in the test tube. The manganese dioxide acts as a catalyst, reducing the temperature at which the potassium chlorate decomposes.
Uses of oxygen
Oxygen is used in steelmaking (by the Bessemer process), welding, the manufacture of industrial chemicals, and in apparatus for breathing (oxygen masks) and resuscitation (oxygen tents). Liquid oxygen is used as an oxidizer in many large rocket engines.
Oxygen on Earth
After the Earth formed, about 4.5 billion years ago, it was continuously struck by comets and asteroids for hundreds of millions of years. After this incessant rain of impacting objects stopped, life appears to have established itself comparatively quickly, perhaps within 10 to 100 million years. As soon as life began on Earth, it started to disturb the atmosphere's chemical equilibrium. Oxygen built up slowly in the atmosphere over the next three billion years because it was a waste product expelled by some single-celled organisms. Then, around 600 million years ago, multicellular creatures developed and the oxygen content of the atmosphere rose suddenly. This trend was assisted by plants, which established themselves on the continents around 450 million years ago. Concurrently, evolution produced animals that breathed oxygen, instead of excreting it as a waste product. Without life, the oxygen content of the atmosphere would have sunk to extremely low levels.
Oxygen in the universe
Oxygen is the most abundant heavy element in the universe, and the third most abundant element overall, after hydrogen and helium. It is produced by massive stars – those born with a mass of over 8 solar masses – which eject the element into interstellar space when they explode. Oxygen burning is the fusion of oxygen into the even heavier elements silicon and sulfur.