Ozone () (or trioxygen) is an
inorganic
An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bondsthat is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''.
Inor ...
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
with the
chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
. It is a pale blue gas with a distinctively pungent smell. It is an
allotrope of
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
that is much less stable than the
diatomic
Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...
allotrope , breaking down in the lower atmosphere to (
dioxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are:
* Ato ...
). Ozone is formed from dioxygen by the action of
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
(UV) light and electrical discharges within the
Earth's atmosphere. It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the
ozone layer
The ozone layer or ozone shield is a region of Earth's stratosphere that absorption (electromagnetic radiation), absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the a ...
of the
stratosphere
The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...
, which absorbs most of the
Sun's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of
chlorine
Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
, and detectable by many people at concentrations of as little as in air. Ozone's O
3 structure
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
was determined in 1865. The molecule was later proven to have a bent structure and to be weakly
diamagnetic. At
standard temperature and pressure, ozone is a pale blue gas that condenses at
cryogenic temperatures to a dark blue
liquid
Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
and finally a violet-black
solid
Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
. Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point.
It is therefore used commercially only in low concentrations.
Ozone is a powerful
oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about . While this makes ozone a potent respiratory hazard and pollutant near
ground level, a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
Nomenclature
The
trivial name ''ozone'' is the most commonly used and
preferred IUPAC name. The systematic names ''2λ
4-trioxidiene'' and ''catena-trioxygen'', valid
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
names, are constructed according to the
substitutive and
additive nomenclatures, respectively. The name ''ozone'' derives from ''ozein'' (ὄζειν), the
Greek neuter present participle for smell, referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as
trioxidane with two hydrogen atoms removed, and as such, ''trioxidanylidene'' may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named ''trioxidanediyl''.
''Trioxidanediyl'' (or ''ozonide'') is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
History
In 1785, Dutch chemist
Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone.
A half century later,
Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of
lightning
Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...
. In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ' () meaning "to smell".
For this reason, Schönbein is generally credited with the discovery of ozone.
He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of
white phosphorus with air is identical.
A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen".
The
chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
for ozone, O
3, was not determined until 1865 by
Jacques-Louis Soret and confirmed by Schönbein in 1867.
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers.
Beaumont, California, had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. "There is quite a different atmosphere
t higher elevationwith enough ozone to sustain the necessary energy
o work, wrote naturalist
Henry Henshaw, working in Hawaii. Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of
halogenated seaweed metabolites
and
dimethyl sulfide.
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873
James Dewar and
John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action".
Schönbein himself reported that chest pains, irritation of the
mucous membranes, and difficulty breathing occurred as a result of inhaling ozone, and small mammals died.
In 1911,
Leonard Hill and
Martin Flack stated in the ''
Proceedings of the Royal Society B'' that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time."
During
World War I
World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
, ozone was tested at
Queen Alexandra Military Hospital in London as a possible
disinfectant
A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than ...
for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with
antiseptics, were found preferable.
Until the 1920s, it was not certain whether small amounts of
oxozone, , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical.
In 1923,
Georg-Maria Schwab (working for his doctoral thesis under
Ernst Hermann Riesenfeld) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis.
Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s.
Physical properties

Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert
non-polar solvents such as
carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At , it condenses to form a dark blue
liquid
Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...
. It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below , it forms a violet-black
solid
Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
.
Ozone has a very specific sharp odour somewhat resembling
chlorine bleach. Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages.
Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly
diamagnetic.
Structure
According to experimental evidence from
microwave spectroscopy, ozone is a bent molecule, with C
2v symmetry
Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...
(similar to the
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
molecule). The O–O distances are . The O–O–O angle is 116.78°. The central atom is ''sp''² hybridized with one lone pair. Ozone is a polar molecule with a
dipole moment of 0.53
D. The molecule can be represented as a
resonance
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
hybrid with two contributing structures, each with a
single bond on one side and
double bond on the other. The arrangement possesses an overall
bond order of 1.5 for both sides. It is
isoelectronic with
the nitrite anion. Naturally occurring ozone can be composed of substituted isotopes (
16O,
17O,
18O). A
cyclic form has been predicted but not observed.
Reactions
Ozone is among the most powerful
oxidizing agents known, far stronger than . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its
half-life Half-life is a mathematical and scientific description of exponential or gradual decay.
Half-life, half life or halflife may also refer to:
Film
* Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang
* ''Half Life: ...
varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour.
:
2 O3 -> 3 O2
This reaction proceeds more rapidly with increasing temperature.
Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10
wt% or higher.
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes.
:
This can be observed as an unwanted reaction in a
Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
With metals
Ozone oxidizes most
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s (except
gold
Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
,
platinum
Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
, and
iridium) into
oxide
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
s of the metals in their highest
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
. For example:
:
With nitrogen and carbon compounds
Ozone oxidizes
nitric oxide to
nitrogen dioxide:
:
NO + O3 -> NO2 + O2
This reaction is accompanied by
chemiluminescence.
The can be further oxidized to
nitrate radical:
:
NO2 + O3 -> NO3 + O2
The formed can react with to form
dinitrogen pentoxide ().
Solid
nitronium perchlorate can be made from , and gases:
:
NO2 + ClO2 + 2 O3 -> NO2ClO4 + 2 O2
Ozone does not react with ammonium
salts, but it oxidizes
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
to
ammonium nitrate
Ammonium nitrate is a chemical compound with the formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly us ...
:
:
2 NH3 + 4 O3 -> NH4NO3 + 4 O2 + H2O
Ozone reacts with
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
to form
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, even at room temperature:
:
C + 2 O3 -> CO2 + 2 O2
With sulfur compounds
Ozone oxidizes
sulfide
Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...
s to
sulfates. For example,
lead(II) sulfide is oxidized to
lead(II) sulfate:
:
PbS + 4 O3 -> PbSO4 + 4 O2
Sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
can be produced from ozone, water and either elemental
sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
or
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
:
:
In the
gas phase, ozone reacts with
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
to form sulfur dioxide:
:
H2S + O3 -> SO2 + H2O
In an
aqueous
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...
solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
:
:
With alkenes and alkynes
Alkenes can be oxidatively cleaved by ozone, in a process called
ozonolysis, giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.

Ozone can also cleave alkynes to form an
acid anhydride or
diketone product. If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
s.
:

Usually ozonolysis is carried out in a solution of
dichloromethane, at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g.
zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
in
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...
or
dimethyl sulfide), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
), carboxylic acids will be formed.
Other substrates
All three
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s of ozone may also react, as in the reaction of
tin(II) chloride with
hydrochloric acid and ozone:
:
3 SnCl2 + 6 HCl + O3 -> 3 SnCl4 + 3 H2O
Iodine perchlorate can be made by treating
iodine dissolved in cold
anhydrous perchloric acid with ozone:
:
I2 + 6 HClO4 + O3 -> 2 I(ClO4)3 + 3 H2O
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination:
:
2KI + O3 + H2O -> 2KOH + O2 + I2
Combustion
Ozone can be used for
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
reactions and combustible gases; ozone provides higher temperatures than burning in
dioxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (). Others are:
* Ato ...
(). The following is a reaction for the combustion of
carbon subnitride which can also cause higher temperatures:
:
3 C4N2 + 4 O3 -> 12 CO + 3 N2
Ozone can react at cryogenic temperatures. At , atomic
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
reacts with liquid ozone to form a hydrogen
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
radical, which
dimerizes:
:
Ozone decomposition
Types of ozone decomposition
Ozone is a toxic substance,
commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The
catalytic
Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are
manganese dioxide, transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of
chlorine
Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
(Cl), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
* Thermal decomposition, in which the ozone is decomposed using only the action of heat. The problem is that this type of decomposition is very slow with temperatures below 250 °C. However, the decomposition rate can be increased working with higher temperatures but this would involve a high energy cost.
* Photochemical decomposition, which consists of radiating ozone with ultraviolet radiation (UV) and it gives rise to oxygen and radical peroxide.
Kinetics of ozone decomposition into molecular oxygen
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two
elementary reactions that finally lead to molecular oxygen, and this means that the reaction order and the
rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction:
2 O3 -> 3 O2
Rate law (observed):
where
is the observed
rate constant and
is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a
reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction
O3 -> O2 + O
Step 2: Bimolecular reaction
O3 + O -> 2 O2
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
: