Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group 16  Melting point −218.79°C, −361.82°F, 54.36 K 
Period Boiling point −182.962°C, −297.332°F, 90.188 K 
Block Density (g cm−3) 0.001308 
Atomic number Relative atomic mass 15.999  
State at 20°C Gas  Key isotopes 16
Electron configuration [He] 2s22p4  CAS number 7782-44-7 
ChemSpider ID 140526 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image represents the fundamental importance of the element in air and, when bonded to hydrogen, in water.
A colourless, odourless gas.
The greatest commercial use of oxygen gas is in the steel industry. Large quantities are also used in the manufacture of a wide range of chemicals including nitric acid and hydrogen peroxide. It is also used to make epoxyethane (ethylene oxide), used as antifreeze and to make polyester, and chloroethene, the precursor to PVC.

Oxygen gas is used for oxy-acetylene welding and cutting of metals. A growing use is in the treatment of sewage and of effluent from industry.
Biological role
Oxygen first appeared in the Earth’s atmosphere around 2 billion years ago, accumulating from the photosynthesis of blue-green algae. Photosynthesis uses energy from the sun to split water into oxygen and hydrogen. The oxygen passes into the atmosphere and the hydrogen joins with carbon dioxide to produce biomass.

When living things need energy they take in oxygen for respiration. The oxygen returns to the atmosphere in the form of carbon dioxide.

Oxygen gas is fairly soluble in water, which makes aerobic life in rivers, lakes and oceans possible.
Natural abundance
Oxygen makes up 21% of the atmosphere by volume. This is halfway between 17% (below which breathing for unacclimatised people becomes difficult) and 25% (above which many organic compounds are highly flammable). The element and its compounds make up 49.2% by mass of the Earth’s crust, and about two-thirds of the human body.

There are two key methods used to obtain oxygen gas. The first is by the distillation of liquid air. The second is to pass clean, dry air through a zeolite that absorbs nitrogen and leaves oxygen. A newer method, which gives oxygen of a higher purity, is to pass air over a partially permeable ceramic membrane.

In the laboratory it can be prepared by the electrolysis of water or by adding a manganese(IV) oxide catalyst to aqueous hydrogen peroxide.
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In 1608, Cornelius Drebbel had shown that heating saltpetre (potassium nitrate, KNO3) released a gas. This was oxygen although it was not identified as such.

The credit for discovering oxygen is now shared by three chemists: an Englishman, a Swede, and a Frenchman. Joseph Priestley was the first to publish an account of oxygen, having made it in 1774 by focussing sunlight on to mercuric oxide (HgO), and collecting the gas which came off. He noted that a candle burned more brightly in it and that it made breathing easier. Unknown to Priestly, Carl Wilhelm Scheele had produced oxygen in June 1771. He had written an account of his discovery but it was not published until 1777. Antoine Lavoisier also claimed to have discovered oxygen, and he proposed that the new gas be called oxy-gène, meaning acid-forming, because he thought it was the basis of all acids.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 1.52 Covalent radius (Å) 0.64
Electron affinity (kJ mol−1) 140.976 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)

Bond enthalpy (kJ mol−1)
A measure of how much energy is needed to break all of the bonds of the same type in one mole of gaseous molecules.

Bond enthalpies

Covalent bond Enthalpy (kJ mol−1) Found in
H–O 462.8 H2O
O–O 146 H2O2
O=O 498.3 O2
O=S 435 SO3
O–Si 452 SiO2
C–O 357.7 general
C=O 803 CO2
C=O 695 HCHO
C=O 736 aldehydes
C=O 749 ketones
C–O 335.6 CH3OH


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states -1, -2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  16O 15.995 99.757
  17O 16.999 0.038
  18O 17.999 0.205


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk Unknown
Crustal abundance (ppm) 461000
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) Unknown
Reserve distribution (%) Unknown
Top 3 producers
  • Unknown
Top 3 reserve holders
  • Unknown
Political stability of top producer Unknown
Political stability of top reserve holder Unknown


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
918 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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Listen to Oxygen Podcast
Transcript :

Chemistry in its element: oxygen


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

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Chris Smith

Hello! And welcome to Chemistry in its element, where we take a look at the stories behind the elements that make up the world around us. I'm Chris Smith. This week, we are continuing our tour of the periodic table with a lung full of a gas that we can't do without. It protects us from solar radiation, it keeps us alive and by helping things to burn, it also keeps us warm. It is of course oxygen. And to tell its story, here's Mark Peplow.

Mark Peplow

Little did those humble cyanobacteria realize what they were doing when two and a half billion years ago, they started to build up their own reserves of energy-rich chemicals, by combining water and carbon dioxide. Powered by sunlight, they spent the next two billion years terraforming our entire planet with the waste products of their photosynthesis, a rather toxic gas called oxygen. In fact, those industrious bugs are ultimately responsible for the diversity of life, we see around us today. 

Oxygen accounts for about 23% of the atmosphere's mass with pairs of oxygen atoms stuck together to make dioxygen molecules, but it's not just in the air, we breathe. Overall, it's the most abundant element on the earth's surface and the third most abundant in the universe after hydrogen and helium. Our planet's rocks are about 46% oxygen by weight, much of it in the form of silicon dioxide, which we know most commonly as sand. And many of the metals we mine from the Earth's crust are also found as their oxides, aluminium in bauxite or iron in hematite, while carbonates such as limestone are also largely made of oxygen and the oceans are of course about 86% oxygen, connected to hydrogen as good old H2O, just about the most perfect solvent you can imagine for biochemistry. 

Oxygen is also in virtually every molecule in your body including fats, carbohydrates and DNA. In particular, it's the atom that links together the phosphate groups in the energy-carrying molecule ATP. Oxygen is obviously pretty useful for keeping us going, but is also widely used in industry as an oxidant, where it can give up some of that solar energy captured by plant and those cyanobacteria. A stream of oxygen can push the temperature of a blast furnace over 2000 degrees and it allows an oxyacetylene torch to cut straight through metal. The space shuttle is carried into space on an incredible force produced when liquid oxygen and liquid hydrogen combine to make water.

So who first noticed this ubiquitous stuff? There's certainly some debate about who first identified oxygen as an element, partly because at the time the precise definition of an element still hadn't really been pinned down. English chemist, Joseph Priestley certainly isolated oxygen gas in the 1770s, although he tried to define it as dephlogisticated air. Phlogiston was then thought to be some kind of primordial substance that was the root cause of combustion. Swedish chemist, Carl Wilhelm Scheele was a fan of phlogiston too and probably discovered oxygen before Priestly did. But it was Antoine Lavoisier, sometimes called the father of modern chemistry, who was the first to truly identify oxygen as an element and in doing so, he really helped to firm up the definition that an element is something that cannot be broken down by any kind of chemical analysis. This also helped him to kill off the phlogiston theory, which was a crucial step in the evolution of chemistry.

Oxygen isn't only about the dioxygen molecules that sustain us. There is another form, trioxygen, also known as ozone and it's also pretty important in the upper reaches of the atmosphere, is responsible for filtering out harmful ultraviolet rays, but unfortunately, ozone is also pretty toxic. So it's bad news that tons of the gas are produced by the reactions between hydrocarbons and nitrogen oxides churned out by cars every day. If only we could transplant the stuff, straight up into the stratosphere! Now ozone is normally spread so thinly in the air, that you can't see its pale blue colour and oxygen gas is colourless unless you liquefy it, but there is one place where you can see the gas in all its glory. The aurora or polar lights, where particles from the solar wind slam into oxygen molecules in the upper atmosphere to produce the swirling green and red colours that have entranced humans for millennia.

Chris Smith

So why life is a gas, that was Mark Peplow revealing the secrets of the element that we can't live without. Next time on Chemistry in its element, Johnny Ball joins us to tell the story of a chemical that's craved by Olympic athletes, makes good hi-five connectors and is also a favourite for fillings. And that's in teeth, not pies.

Johnny Ball

Today one gram can be beaten into a square meter sheet just 230 atoms thick, one cubic centimetre would make a sheet 18 square meters, 1 gram could be drawn out to make 165 meters of wire just 1/200th of a millimetre thick. The gold colour in Buckingham Palace fence is actually gold; gold covered because it lasts 30 years; whereas gold paint which actually contains no gold at all lasts in tip-top condition only a year or so.

Chris Smith

So all that glitters isn't gold, but some is, and you can find out why on next week's Chemistry in its element. I'm Chris Smith, thanks for listening. See you next time.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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C5e Demonstrate that dissolving, mixing and change of state are reversible.
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Education in Chemistry
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The reaction between aluminium and iodine is catalysed by water. This is a spectacular demonstration as clouds of purple iodine vapour are produced.

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Visual Elements images and videos
© Murray Robertson 2011.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J.J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions(version 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.
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