Skip to Content


« Back to Glossary Index

**Historical Development and Discovery**:
– Early experiments by Philo of Byzantium, Leonardo da Vinci, Robert Boyle, and John Mayow on combustion and air relationship.
– Phlogiston theory and its proponents like Robert Hooke, Ole Borch, and Pierre Bayen.
– Discovery of oxygen credited to Joseph Priestley, Michael Sendivogius, and contributions from Scheele and Lavoisier.
– Lavoisier’s role in discrediting the phlogiston theory and correctly explaining combustion.
– Later historical developments including Dalton’s atomic hypothesis, commercial oxygen production methods, and discovery of liquid oxygen.

**Chemical and Physical Properties**:
– Oxygen’s chemical properties as a reactive nonmetal and oxidizing agent.
– Abundance of oxygen in Earth’s crust and atmosphere, constituting 20.95% of the air.
– Role of oxygen in cellular respiration, energy extraction, and various industrial applications.
– Physical properties of oxygen including solubility in water, freezing and condensing points, and spectroscopic characteristics.
– Allotropes of oxygen such as dioxygen, ozone, tetraoxygen, and metallic phases, with their unique properties and potential applications.

**Applications and Environmental Impact**:
– Diverse uses of oxygen in steel production, welding, rocket propellant, life support systems, and more.
– Importance of oxygen in aircraft, submarines, spaceflight, and environmental processes like photosynthesis and ozone layer protection.
– Impact of fossil fuel burning on global oxygen levels and the trend of decreasing oxygen in the atmosphere.
– Environmental significance of oxygen in the ozone layer, as a pollutant near the Earth’s surface, and its role in various ecosystems.
– Unique properties of liquid oxygen, its handling considerations, and innovations in its applications like welding and rocket propulsion.

**Isotopes, Stellar Origin, and Analysis**:
– Composition of oxygen isotopes and their stellar origin in massive stars and hydrogen burning processes.
– Use of oxygen isotopes in paleoclimatology, ice core analysis, and remote sensing applications.
– Biological production of oxygen through photosynthesis by organisms like green algae, cyanobacteria, and terrestrial plants.
– Role of oxygen in the oxygen cycle, including production, conversion of carbon dioxide, and movement within the atmosphere, biosphere, and lithosphere.
– Spectrophotometric properties of oxygen, its absorption bands, and applications in monitoring plant health and climate history.

**Photosynthesis, Respiration, and Role in Earth’s Atmosphere**:
– Processes of photosynthesis and respiration involving oxygen production, carbon dioxide conversion, and energy generation.
– Contribution of green algae, cyanobacteria, and oceans to Earth’s atmospheric oxygen.
– Importance of the oxygen cycle in maintaining Earth’s high oxygen concentration and supporting ocean life.
– Effects of water pollution on oxygen content through eutrophication and monitoring methods like biochemical oxygen demand.
– Significance of oxygen as the second most common component in Earth’s atmosphere and its crucial role in sustaining life on the planet.

Oxygen (Wikipedia)

Oxygen is a chemical element; it has symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, and after hydrogen and helium, it is the third-most abundant element in the universe. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O
. Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. Oxygen makes up almost half of the Earth's crust in the form of oxides.

Oxygen, 8O
Gas discharge tube filled with oxygen O2.
Oxygen-filled discharge tube glowing purple.
AllotropesO2, O3 (ozone) and more (see Allotropes of oxygen)
Appearancegas: colorless
liquid and solid: pale blue
Standard atomic weight Ar°(O)
  • [15.9990315.99977]
  • 15.999±0.001 (abridged)
in the Earth's crust461000 ppm
Oxygen in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Atomic number (Z)8
Groupgroup 16 (chalcogens)
Periodperiod 2
Block  p-block
Electron configuration[He] 2s2 2p4
Electrons per shell2, 6
Physical properties
Phase at STPgas
Melting point(O2) 54.36 K ​(−218.79 °C, ​−361.82 °F)
Boiling point(O2) 90.188 K ​(−182.962 °C, ​−297.332 °F)
Density (at STP)1.429 g/L
when liquid (at b.p.)1.141 g/cm3
Triple point54.361 K, ​0.1463 kPa
Critical point154.581 K, 5.043 MPa
Heat of fusion(O2) 0.444 kJ/mol
Heat of vaporization(O2) 6.82 kJ/mol
Molar heat capacity(O2) 29.378 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K)       61 73 90
Atomic properties
Oxidation states−2, −1, 0, +1, +2
ElectronegativityPauling scale: 3.44
Ionization energies
  • 1st: 1313.9 kJ/mol
  • 2nd: 3388.3 kJ/mol
  • 3rd: 5300.5 kJ/mol
  • (more)
Covalent radius66±2 pm
Van der Waals radius152 pm
Color lines in a spectral range
Spectral lines of oxygen
Other properties
Natural occurrenceprimordial
Crystal structurecubic (cP16)
Lattice constant
Cubic crystal structure for oxygen
a = 678.28 pm (at t.p.)
Thermal conductivity26.58×10−3  W/(m⋅K)
Magnetic orderingparamagnetic
Molar magnetic susceptibility+3449.0×10−6 cm3/mol (293 K)
Speed of sound330 m/s (gas, at 27 °C)
CAS Number7782-44-7
DiscoveryMichael Sendivogius
Carl Wilhelm Scheele (1604, 1771)
Named byAntoine Lavoisier (1777)
Isotopes of oxygen
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
15O trace 122.266 s β+100% 15N
16O 99.8% stable
17O 0.0380% stable
18O 0.205% stable
 Category: Oxygen
| references

All plants, animals, and fungi need oxygen for cellular respiration, which extracts energy by the reaction of oxygen with molecules derived from food and produces carbon dioxide as a waste product. In tetrapods breathing brings oxygen into the lungs where gas exchange takes place, carbon dioxide diffuses out of the blood, and oxygen diffuses into the blood. The body's circulatory system transports the oxygen to the cells, where cellular respiration takes place.

Many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form (allotrope) of oxygen, ozone (O
), strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of smog and thus a pollutant.

Oxygen was isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774. Priority is often given for Priestley because his work was published first. Priestley, however, called oxygen "dephlogisticated air", and did not recognize it as a chemical element. The name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and correctly characterized the role it plays in combustion.

Common uses of oxygen include production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy, and life support systems in aircraft, submarines, spaceflight and diving.

« Back to Glossary Index