Chemical element · reference page

Oxygen

Oxygen is element 8 (O), located in period 2, group 16 and the p block. At room temperature its reference phase is gas. Its electron configuration is [He] 2s2 2p4.

Atomic number8

Standard atomic weight[15.99903, 15.99977]

Group16

Period2

Blockp

Electron configuration[He] 2s2 2p4

Electronic structure

What controls its chemistry

Group 16 atoms have six valence electrons. Bonding often involves gaining or sharing two electrons, while the heavier members can access a wider range of oxidation states.

For oxygen, this outer-shell structure helps explain the importance of oxides, sulfides, oxoanions and covalent compounds across the chalcogen family.

Chemical forms

Compounds, ions and materials

Elemental oxygen is mainly O₂, with ozone O₃ as another molecular form. In compounds, oxygen forms oxides, hydroxides, oxoanions and covalent bonds throughout mineral and biological chemistry. The −2 oxidation state is especially common, with important exceptions such as peroxides.

Physical behaviour

State and phase changes

The reference phase at room temperature is gas. The listed density is 1.429 g/L. The melting point is 54.37 K (-218.8 °C). The boiling point is 90.19 K (-183 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where oxygen is found and used

In practice, oxygen is encountered in respiration, metallurgy, steel and water treatment.

In practice

Examples of real uses

Respiration

Oxygen chemistry supports aerobic respiration by accepting electrons in cellular metabolism.

Metallurgy

Oxygen is used in extraction, refining, alloying or heat-treatment processes that control the properties of metals.

Steel

Oxygen changes the properties of steels or appears in steelmaking chemistry and alloy design.

Water treatment

Oxygen is used in disinfection, purification, oxidation or treatment chemistry.

Periodic position

What its neighbours tell us

Oxygen is in period 2 and group 16. In the neighbourhood shown here, it lies between nitrogen and fluorine in the same period; sulfur is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationdiatomic nonmetal

Phase at room temperatureGas

Electron shells2 · 6

Density1.429 g/L

Melting point54.37 K (-218.8 °C)

Boiling point90.19 K (-183 °C)

Pauling electronegativity3.44

First ionisation energy1 314 kJ/mol

Electron affinity141 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of oxygen is associated with Carl Wilhelm Scheele. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is given as the interval [15.99903, 15.99977]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Oxygen has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with oxygen depend on chemical form, dose and route of exposure. Pure oxygen, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.