Oxygen: the essentials

Oxygen is a Group 16 element. While about one fifth of the atmosphere is oxygen gas, the atmosphere of Mars contains only about 0.15% oxygen. Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, one process responsible for stellar energy production. Oxygen in excited states is responsible for the bright red and yellow-green colours of the aurora. About two thirds of the human body, and nine tenths of water, is oxygen. The gas is colourless, odourless, and tasteless. Liquid and solid oxygen are pale blue (see picture above) and strongly paramagnetic (contains unpaired electrons).

Ozone (O3) is the second allotrope of oxygen. It is formed from electrical discharges or ultraviolet light acting on O2. It is an important component of the atmosphere (in total amounting to the equivalent of a layer about 3 mm thick at ordinary pressures and temperatures) which is vital in preventing harmful ultraviolet rays of the sun from reaching the earth's surface. Aerosols in the atmosphere have a detrimental effect on the ozone layer. Large holes in the ozone layer are forming over the polar regions and these are increasing in size annually. Paradoxically, ozone is toxic! Undiluted ozone is bluish in colour. Liquid ozone is bluish-black, and solid ozone is violet-black.

Oxygen is very reactive and oxides of most elements are known. It is essential for respiration of all plants and animals and for most types of combustion.

Oxygen occurs as two allotropes, dioxygen and trioxygen (ozone). Dioxygen, O2, is a diatomic colourless oxidizing gas but a pale blue liquid upon condensation. It is paramagnetic (that is, possesses unpaired electrons) as a result of two unpaired electrons in the two O2 π* orbitals. It makes up about a fifth of the earth's atmosphere.

Ozone, O3, is a very reactive blue gas characterized by a pungent "electrical" smell (the name ozone derives from the Greek ozein, to smell). Ozone molecules are bent with a bond angle of about 117° in the gas phase. Ozone condenses to a blue-black liquid (boiling point -111.9°C and further cooling results in a dark violet-black solid (melting point -192.5°C.

Ozone is unstable thermodynamcally with respect to dioxygen but only converts to O2 slowly in the absence of a catalyst.

2O3(g) → 3O2(g)

ozone crystal structure
Arrangements of ozone molecules, O3, in the solid state.

Ozone is present in the atmosphere as well, but at amuch lower level than O2. Ozone is a key component of the upper atmosphere because it has the ability to absorb ultraviolet light in the 220-290 nm range, so preventing harmful rays at those wavelengths reaching the earth's surface. These days, various factors deplete the ozone layer in the upper atmosphere and the "ozone holes" in the northern and southern polar regions are of particular concern.

Oxygen: historical information

Oxygen was discovered by Joseph Priestley, Carl Scheele in 1774 at England and Sweden. Origin of name: from the Greek words "oxy genes" meaning "acid" (sharp) and "forming" (acid former).

Leonardo da Vinci suggested that air consists of at least two different gases. Before then, air was felt to be an element in its own right. He was also aware that one of these gases supported both flames and life. Oxygen was prepared by several workers before 1772 but these workers did not recognise it as an element. Joseph Priestley is generally credited with its discovery (who made oxygen by heating lead or mercury oxides), but Carl Wilhelm Scheele also reported it independently.

The behaviour of oxygen and nitrogen as components of air led to the advancement of the phlogiston theory of combustion, which influenced chemists for a century or so, and which delayed an understanding of the nature of air for many years.

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for oxygen is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Dalton's symbol for oxygen

Oxygen around us Read more »

Oxygen atoms are present in water (H2O) and water is essential to all life. Oxygen and is present in many organic compounds. Most organisms use oxygen for respiration. While oxygen (O2) is necessary for life, oxygen as ozone (O3) is highly toxic. On the other hand, ozone is an important component of the atmosphere (the ozone layer) and helps to shield us from harmful ultraviolet rays from the sun.

Oxygen gas (dioxygen, O2) makes up 21% of the atmosphere by volume. It is the most abundant element in the earth's crust (followed by silicon). It is present in any number of minerals and makes up most of the weight of water. Oxygen is the third most abundant element found in the sun, and is involved in the carbon-nitrogen cycle, one of the processes responsible for stellar energy production. The atmosphere of Mars contains about 0.15% oxygen.

Abundances for cobalt in a number of different environments. More abundance data »
Location ppb by weight ppb by atoms Links
Universe 10000000 800000 Chemical elements abundance by weight in the universe on a miniature periodic table spark table
Crustal rocks 460000000 600000000 Chemical elements abundance by weight in the earth's crust on a miniature periodic table spark table
Human 610000000 ppb by weight 240000000 atoms relative to C = 1000000 Chemical elements abundance by weight in humans on a miniature periodic table spark table

Physical properties Read more »

Heat properties Read more »

Crystal structure Read more »

The solid state structure of oxygen is: monoclinic.

Oxygen: orbital properties Read more »

Oxygen atoms have 8 electrons and the shell structure is 2.6. The ground state electronic configuration of neutral Oxygen is [He].2s2.2p4 and the term symbol of Oxygen is 3P2.


Isolation: there is not normally any need ot make oxygen in the laboratory as it is readily available commercially or through in-house air liquefaction plants. However the decomposition of potassium chlorate is one route to O2 and decomposition of potassium permanganate is another. In addition, electrolysis of KOH using nickel electrodes gives clean oxygen.

2KClO3 (400°C) → 2KCl + 3O2

2KMnO4 (214°C) → K2MnO4 + MnO2 + O2

Ozone (O3), the other allotrope of oxygen, is made by silent electric discharge through oxygen flowing through a cooled system. This can give up to a10% proportion of ozone and the ozone is purified by fractional liquefaction (with care!).

Oxygen isotopes Read more »

All three Oxygen isotopes have medical applications. O-16 is used in the production of radioactive N-13 which is used for PET imaging and myocardial perfusion. O-17 can be used as a tracer in the study of cerebral oxygen utilization. Large quantities of O-18 are used for the production of F-18. F-18 is used to produce 2-[18F]fluoro-2-deoxyglucose (FDG) which is used as a tracer in positron emission tomography or PET. Millions of these FDG-PET medical procedures are performed annually to investigate a range of diseases in various human organs.

Table. Stables isotopes of cobalt.
Isotope Mass
(atom %)
spin (I)
moment (μ/μN)
16O 15.99491463 (5) 99.757 (16) 0 0
17O 16.9991312 (4) 0.038 (1) 5/2 -1.89380
18O 17.9991603 (9) 0.205 (14) 0 0

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