Triplet oxygen

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Triplet oxygen
CAS number 7782-44-7 YesY
EC number 231-956-9
UN number 1072
KEGG D00003 YesY
MeSH Oxygen
RTECS number RS2060000
ATC code V03AN01
Jmol-3D images Image 1
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Molecular formula O2
Molar mass 32.00 g mol−1
Appearance Colorless gas
Melting point −218.2 °C; −360.7 °F; 55.0 K
Boiling point −183.2 °C; −297.7 °F; 90.0 K
Molecular shape Linear
Dipole moment 0 D
205.152 J K-1 mol-1
0 kJ mol-1
EU classification Oxidizing Agent O
R-phrases R8
S-phrases S17
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Triplet oxygen (systematically named dioxidanediyl and dioxygen(2•)) is an inorganic chemical in a ground state, with the chemical formula O2•
. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals. These orbitals are classified as antibonding, so the O-O bond is weaker than the N-N bond in molecular nitrogen, where all bonding molecular orbitals are filled.


The valence orbitals of molecular oxygen (middle); in the ground state, the electrons in the π* orbitals have their spins parallel.

The s=1/2 spins of the two electrons in degenerate orbitals gives rise to 2x2=4 independent spin states in total. Exchange interaction splits these into a singlet state (total spin S=0) and a set of 3 degenerate triplet states (S=1). In agreement with Hund's rules, the triplet states are energetically more favorable, and the ground state of the molecule carries an electron spin of S=1. Excitation to the S=0 state results in much more reactive, metastable singlet oxygen.

Because the molecule in its ground state has a non-zero spin magnetic moment, oxygen is paramagnetic; i.e., it can be attracted to the poles of a magnet. The Lewis structure O=O does not accurately represent the diradical nature of molecular oxygen; molecular orbital theory must be used to adequately account for the unpaired electrons. Triplet oxygen is better described by a bond order of one and two halves instead of two to better reflect its unpaired bonding electrons. This allows for easier reasoning of the bond length.


The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet state. Triplet oxygen will, however, readily react with molecules in a doublet state, such as radicals, to form a new radical. Conservation of spin quantum number would require a triplet transition state in a reaction of triplet oxygen with a closed shell (a molecule in a singlet state). The extra energy required is sufficient to prevent direct reaction at ambient temperatures with all but the most reactive substrates, e.g. white phosphorus. At higher temperatures or in the presence of suitable catalysts the reaction proceeds more readily. For instance, most flammable substances are characterised by an autoignition temperature at which they will undergo combustion in air without an external flame or spark.


Singlet oxygen, where the electron spins are opposed in a higher energy state, is many times more reactive than triplet oxygen, and extremely hazardous to organic matter. It is used for the extermination of heavy infestations of persistent pests in houses and buildings.


  1. ^ a b "Triplet Dioxygen (CHEBI:27140)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 

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