Oxygen Facts

Oxygen is the eighth element of the periodic table. It is atomic number 8 with element symbol O. Oxygen is best known as the element we need in order to breathe, but it has many other interesting properties. These oxygen facts contain chemical and physical data along with general information and history.

Element cell for Oxygen
Oxygen periodic table tile

Basic Oxygen Facts

Name: Oxygen

Atomic Number: 8

Element Symbol: O

Group: 16

Period: 2

Block: p

Element Family: nonmetal

Atomic Mass: [15.999 03; 15.999 77] IUPAC guidelines. For single value, use 15.9994.

Electron Configuration: [He]2s22p(shorthand) or 1s22s22p(full)

Discovery: Carl Wilhelm Scheele in 1773 and Joseph Priestley in 1774. Priestley is often given priority because his work was published first. Both men noticed heating particular oxides released a gas which caused candles to burn brighter. Priestley went a little bit further by placing a mouse in a jar of oxygen. He expected it to survive only a few minutes before suffocating. Instead, the mouse was still alive after an hour in the jar.

Name Origin: French chemist Antoine Lavoisier named the element oxygene from the Greek words oxys meaning “acid” and genes meaning “forming”. At the time, it was believed all acids required oxygen to part of the reaction.


Natural oxygen is comprised of two stable isotopes: 16O, 17O and 18O. Radioactive isotopes have been produced under laboratory conditions ranging from 12O to 24O.

Oxygen-16 is a stable isotope containing 8 neutrons. 99.757% of natural oxygen is oxygen-16.

Oxygen-17 is a stable isotope containing 9 neutrons. 0.038% of natural oxygen is oxygen-17.

Oxygen-18 is a stable isotope containing 9 neutrons. 0.205% of natural oxygen is oxygen-18.

Liquid Oxygen
Liquid oxygen has a distinctive blue color. Credit: Dr. Warwick Hillier / Australian National University

Physical Data

Density: 0.001308 g/cm3

Melting Point: 54.36 K ​(−218.79 °C, ​−361.82 °F)

Boiling Point: 90.188 K ​(−182.962 °C, ​−297.332 °F)

Triple Point: 54.361 K at ​0.1463 kPa

Critical Point: 154.581 K at 5.043 MPa

State at 20ºC: Gas

Heat of Fusion: 0.444 kJ/mol for O2

Heat of Vaporization: 6.82 kJ/mol for O2

Molar Heat Capacity: 29.378 J/mol·K for O2

Oxygen atom
Electron configuration of an oxygen atom.

Atomic Data

Atomic Radius: 1.52 Å

Covalent Radius: 0.64 Å

Van der Waals Radius:  1.55 Å

Electron Affinity: 140.976 kJ/mol

Electronegativity: (Pauling scale): 3.44

1st Ionization Energy: 1313.942 kJ/mol

2nd Ionization Energy: 3388.671 kJ/mol

3rd Ionization Energy: 5300.47 kJ/mol

4th Ionization Energy: 7469.271 kJ/mol

5th Ionization Energy: 10989.584 kJ/mol

6th Ionization Energy: 13326.526 kJ/mol

7th Ionization Energy: 71330.65 kJ/mol

8th Ionization Energy: 84078.3 kJ/mol

Oxidation States: -2, -1 (common), +2, +1 (uncommon)

Green Aurora in Iceland (Anne Helmenstine)
Green Aurora in Iceland. The green color is caused by the ionization of oxygen in the upper atmosphere. (Anne Helmenstine)

Fun Oxygen Facts

  • Oxygen is a colorless, tasteless and odorless gas, but liquid oxygen has a distinct blue color.
  • Oxygen makes up about 21% of the air you breathe by volume.
  • Nearly half the Earth’s crust is oxygen by mass.
  • Oxygen is the most abundant element in the human body accounting for 65% of your weight.
  • All animals and plants require oxygen to live.
  • Oxygen is not flammable. However, it supports combustion of other materials and should not be stored near flammable materials.
  • Oxygen readily dissolves in water. This property makes life underwater possible.
  • Oxygen gas is used extensively by the steel industry to make steel. It is also used to make hydrogen peroxide and nitric acid. The compound epoxyethane is produced to make the common polymers polyester and PVC (polyvinylchloride)
  • The green waves of color in the auroras is caused by the ionization of oxygen in the upper atmosphere.
  • Oxygen is rarely found alone in nature. Oxygen bonds to itself to form diatomic O2. Ozone is another pure oxygen molecule with formula O3.
  • Ozone is vital to absorb harmful ultraviolet radiation from the Sun in the upper atmosphere. Closer to the ground, however, ozone is considered a pollutant and is hazardous to life.
  • Oxygen was the standard for measuring the atomic weights of other elements until 1961. It was replaced by the mass of carbon-12.

Learn more about elements on the periodic table.