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colorless gas; light blue liquid. Oxygen bubbles go up in this picture of liquid oxygen.

Light reflection lines of oxygen
General properties
Name, symbol, number oxygen, O, 8
Pronunciation /ˈɒksɨən/ OK-si-jən
Element category nonmetal, chalcogen
Group, period, block 162, p
Standard atomic weight 15.999(1)g·mol−1
Electron configuration [He] 2s2 2p4
Electrons per shell 2, 6 (Image)
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
1.429 g/L
Liquid density at b.p. 1.141 g·cm−3
Melting point 54.36 K, -218.79 °C, -361.82 °F
Boiling point 90.20 K, -182.95 °C, -297.31 °F
Critical point 154.59 K, 5.043 MPa
Heat of fusion (O2) 0.444 kJ·mol−1
Heat of vaporization (O2) 6.82 kJ·mol−1
Specific heat capacity (25 °C) (O2)
29.378 J·mol−1·K−1
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, −1, −2
Electronegativity 3.44 (Pauling scale)
Ionization energies
1st: 1313.9 kJ·mol−1
2nd: 3388.3 kJ·mol−1
3rd: 5300.5 kJ·mol−1
Covalent radius 66±2 pm
Van der Waals radius 152 pm
Crystal structure cubic
Magnetic ordering paramagnetic
Thermal conductivity (300 K) 26.58x10-3  W·m−1·K−1
Speed of sound (gas, 27 °C) 330 m/s
CAS registry number 7782-44-7
Most stable isotopes
Main article: Isotopes of oxygen
iso NA half-life DM DE (MeV) DP
16O 99.76% 16O is stable with 8 neutrons
17O 0.039% 17O is stable with 9 neutrons
18O 0.201% 18O is stable with 10 neutrons

Oxygen (O) is a chemical element. In nature, oxygen is a gas with no color or smell. Oxygen is a very important element because it is a part of the air people breathe and the water people drink. Because of this, oxygen supports life. Many living things (including humans) need oxygen to live and breathe, though it is poisonous to some forms of life. In liquid state of matter, oxygen is light blue.[1][2][3]

Discovery of oxygen

Oxygen was initially discovered in 1772 by Carl Wilhelm Scheele. Then Joseph Priestley also discovered it two years later and spread the news before Scheele. This made many people think that Priestley discovered oxygen first. The word oxygen comes from Greek words: oxus (acid) and gennan (generate). Oxygen comes to earth from all parts of the universe in meteorites. It is also found in minerals and plants that grow on the earth and other planets.

Oxygen in nature

A drop of water.
Water contains oxygen. When one oxygen atom combines with two hydrogen atoms, they form a molecule of water (also written as H2O). This water is required by all living things to live.

Air also contains oxygen. Air is made of many gases that are mixed together. Approximately 21% of the air is oxygen. Oxygen is especially important because all animals use oxygen to get energy from their food.

Combustion of wood in a match.

Uses of oxygen

Oxygen is also what makes burning possible. This is called combustion. When an object or something burns or combusts, oxygen combines with another substance and releases heat and light. For instance, when you burn wood, the oxygen in air combines with the wood to create fire. This ability of oxygen has many uses. But, it also makes pure-oxygen very dangerous. If pure oxygen touches a flame or spark, it can combust and cause great damage. Combustion is used in many ways. For example, when oxygen is mixed with acetylene, it can create a very hot flame. The hot flame is used in welding, where metal is heated and melted together. Also, oxygen is used to create powerful fuels, which also can be used in rockets and jets.

Oxygen can be used in smelting metal from ore.

Oxygen is used in hospitals for killing bacteria. It is also used when a patient has had carbon monoxide poisoning.

Oxygen is used in water treatment to purify the water to make it safe for us to drink.

Production of gas

Pure oxygen can be produced in several ways. In nature, plants produce oxygen by using sunlight, carbon dioxide (another gas) and water. This process is called photosynthesis. Most of the oxygen in the air is produced by photosynthesis.

Oxygen can also be created by humans, using a process called electrolysis. In this process, electricity passes through water. As a result, the water molecules break and release oxygen and hydrogen gas.


  1. "Liquid Oxygen Page". 2003 [last update]. Retrieved 22 May 2012.
  2. "Oxygen Facts - Periodic Table of the Chemical Elements". 2012 [last update]. Retrieved 22 May 2012.
  3. "Lox/LH2". 2011 [last update]. Retrieved 22 May 2012.