ChemSphere
8O15.999

Oxygen

Reactive nonmetal · Period 2 · Group 16 · p-block

The element that breathes, burns and rusts

Atomic mass
15.999 u
State at 25 °C
Gas
Electronegativity
3.44
Melting point
-218.8 °C
Boiling point
-183 °C
Configuration
[He] 2s2 2p4
Building atom…
8 protons 8 neutrons 8 electrons

Oxygen, explained

Overview

Oxygen has six valence electrons ([He]2s2 2p4) and, after fluorine, is the most electronegative element (3.44 on the Pauling scale). It pulls electrons strongly from almost every other element, forming the oxide ion O2- or two polar covalent bonds, and the word oxidation itself comes from oxygen. It is the most abundant element in Earth's crust, about 46% by mass, bound in silicate and oxide minerals, and it makes up 89% of the mass of water.

The O2 molecule has two unpaired electrons, a fact explained by molecular orbital theory rather than simple Lewis structures. As a result, liquid oxygen is pale blue and is attracted to a magnet. Free oxygen makes up 21% of the air and exists only because photosynthesis constantly replenishes it; it began accumulating in the Great Oxidation Event about 2.4 billion years ago. High in the stratosphere, ultraviolet light converts some O2 into ozone, O3, which absorbs harmful UV radiation before it reaches the surface.

Discovery

Carl Wilhelm Scheele produced oxygen around 1771 by heating mercuric oxide and other compounds and called it 'fire air', but his work was not published until 1777. Joseph Priestley independently made it in 1774 by focusing sunlight on mercuric oxide and called it 'dephlogisticated air'. Antoine Lavoisier showed it was an element responsible for combustion and named it oxygène, from the Greek oxys (acid) and genes (forming), wrongly believing it was present in all acids.

Where it’s found
Oxygen is the most abundant element in Earth's crust and makes up 21% of the atmosphere; industrial oxygen is produced by fractional distillation of liquid air.
What it’s used for
  • Basic oxygen steelmaking
  • Medical oxygen therapy
  • Rocket oxidizer
  • Ozone water treatment
  • Welding and cutting
  • Chemical manufacturing
In living things
Oxygen is essential for aerobic life: in mitochondria it accepts electrons at the end of the respiratory chain, allowing cells to extract most of the energy in food. Haemoglobin carries it from the lungs to the tissues.
Safety
Oxygen does not burn itself but makes other materials burn far more fiercely; oil or grease on oxygen fittings can ignite, and oxygen-enriched rooms are a serious fire hazard. Ozone is a lung irritant.

How it compares

Where Oxygen sits among all 118 elements on the trends that shape the table.

  • Electronegativity3.44Pauling

    #2 of 100 elements with data

  • Ionization energy1,310kJ/mol

    #7 of 104 elements with data

  • Atomic radius60pm

    #93 of 95 elements with data

  • Electron affinity141kJ/mol

    #15 of 109 elements with data

  • Density0.001g/cm³

    #110 of 114 elements with data

  • Melting point54.4K

    #106 of 110 elements with data

Density
1.43 g/L
Atomic radius
60 pm
Covalent radius
63 pm
Molar heat
—
Appearance
—
Discovered by
Carl Wilhelm Scheele

Inside the atom

8 electrons arranged in 2 shells: 2 · 6.

Orbital diagram

1s2 2s2 2p4 · shorthand [He] 2s2 2p4

  1. 1s2
  2. 2s2
  3. 2p4

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Oxygen bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. The biggest leap, from electron 6 to 7 (×5.4), is where the next electron has to come from a filled shell much closer to the nucleus.

Signature reactions

The chemistry Oxygen is known for.

  1. Redox · Biochemical

    Aerobic respiration

    Cells oxidize glucose in a series of steps through glycolysis, the citric acid cycle and the electron transport chain. The overall reaction releases about 2800 kJ per mole of glucose, much of it captured as ATP. It is the reverse of the net equation for photosynthesis.

    C6H12O6(aq)+6O2(g)6CO2(g)+6H2O(l)

    Conditions: living cells, enzyme-catalysed

  2. Photochemical

    Ozone formation

    Ultraviolet light splits O2 molecules into single oxygen atoms, which then combine with other O2 molecules to form ozone. The ozone layer absorbs UV-B and UV-C radiation that would otherwise damage DNA. Chlorine from CFCs catalyses ozone destruction, which is why CFCs were banned.

    3O2(g)2O3(g)

    Conditions: UV light in the stratosphere

  3. Decomposition

    Priestley's decomposition of mercuric oxide

    Heating red mercuric oxide releases oxygen gas and leaves droplets of liquid mercury. Priestley found that a candle burned remarkably brightly in the gas, and a mouse lived longer in it than in ordinary air. This experiment was central to Lavoisier's new theory of combustion.

    2HgO(s)2Hg(l)+O2(g)

    Conditions: strong heating

Watch Oxygen react in 3D

Where you’ll meet it

  • Aurora

    Aurora

    The most common green auroral glow is light emitted by excited oxygen atoms about 100-250 km up.

    Photo: Wikimedia Commons
  • Ozone layer

    Ozone layer

    A thin band of ozone in the stratosphere screens out most of the Sun's harmful ultraviolet light.

    Photo: Wikimedia Commons
  • Saturn V

    Saturn V

    The Moon rocket's first stage burned about 1.3 million litres of liquid oxygen with kerosene.

    Photo: Wikimedia Commons
  • Photosynthesis

    Photosynthesis

    Nearly all the oxygen in the air was released by photosynthesis splitting water.

    Photo: Wikimedia Commons

Research