Chlorine monoxide


Chlorine monoxide is a chemical radical with the chemical formula ClO. It plays an important role in the process of ozone depletion. In the stratosphere, chlorine atoms react with ozone molecules to form chlorine monoxide and oxygen.

Chlorine monoxide
Preferred IUPAC name
Chlorine monoxide
Systematic IUPAC name
Other names
Chlorine(II) oxide
  • 7791-21-1 checkY
3D model (JSmol)
  • Interactive image
Abbreviations ClO
  • CHEBI:29314
  • 145843
MeSH Chlorosyl
  • 166686
  • 0EQ5I4TK19 checkY
  • DTXSID90924342 Edit this at Wikidata
  • InChI=1S/ClO/c1-2
  • [O]Cl
Molar mass 51.45 g·mol−1
101.8 kJ/mol[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Cl + O3 → ClO + O2

This reaction causes the depletion of the ozone layer.[1] The resulting ClO radicals can further react:

ClO + O → Cl + O2

regenerating the chlorine radical. In this way, the overall reaction for the decomposition of ozone is catalyzed by chlorine, as ultimately chlorine remains unchanged. The overall reaction is:

O + O3 → 2 O2

This has been a significant impact of the use of CFCs on the upper stratosphere, however many countries have agreed to ban the use of CFCs. The nonreactive nature of CFCs allows them to pass into the stratosphere, where they undergo photo-dissociation to form Cl radicals. These then readily form chlorine monoxide, and this cycle can continue until two radicals react to form dichlorine monoxide, terminating the radical reaction. Because the concentration of CFCs in atmosphere is very low, the probability of a terminating reaction is exceedingly low, meaning each radical can decompose many thousands of molecules of ozone.

Even though the use of CFCs has been banned in many countries, CFCs can stay in the atmosphere for 50 to 500 years. This cause many chlorine radicals to be produced and hence a significant amount of ozone molecules are decomposed before the chlorine radicals are able to react with chlorine monoxide to form dichlorine monoxide.


  1. ^ a b Egon Wiberg; Nils Wiberg; Arnold Frederick Holleman (2001). Inorganic chemistry. Academic Press. p. 462. ISBN 0-12-352651-5.