Ozone

Ball-and-stick Stick Space-filling

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3 atoms, 2 bonds, bent.

IUPAC name: ozone
Molar mass: 47.9970(30) g/mol
Atoms: 3
Shape: bent
SMILES: [O-][O+]=O
InChIKey: CBENFWSGALASAD-UHFFFAOYSA-N
PubChem CID: 24823

Made of

Structural formula of ozone. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

With a third atom joined to the oxygen pair in air, the result shields life in one place and injures lungs in another. Ozone is one of the two allotropes of oxygen, forms of a single element built differently; the other is dioxygen, O₂. Its molar mass is 47.997 g/mol. The gas ranges from colourless to bluish, and cooling turns it into a dark blue liquid. The solid is blue-black in one hazard database and violet-black at Los Alamos.

Making ozone takes energy. OpenStax gives 287 kJ absorbed when three O₂ molecules become two of ozone, supplied by an electrical discharge, heat or ultraviolet light. The product is relatively unstable and returns to dioxygen in two steps: one ozone molecule sheds an oxygen atom, which then takes a second molecule apart. As an oxidiser it releases more heat than dioxygen does, and usually reacts at a lower temperature.

Structure and bonding

The readout counts 3 atoms, 2 bonds; all three atoms are oxygen, drawn in one colour. Labels prints O on all three, so only position tells them apart. In Ball-and-stick, the middle atom holds one neighbour by a single stick and the other by a double one. The chain does not run straight but bends at that middle atom, the shape OpenStax calls bent.

Stick strips the picture down to a V with one doubled arm. Space-filling fuses the three spheres into a single lumpy boomerang whose outline keeps the bend.

The drawing is one resonance structure, one of two or more valid ways to place the electrons over the same arrangement of atoms. It has a single bond, a double bond, a charge of +1 on the middle atom and −1 on the single-bonded end. OpenStax notes that more than one such structure is needed to describe ozone’s bonding. Its general account of resonance says the real electron distribution is an average of the forms, and a molecule never flips from one to another. Neither those charges nor the averaging appear on screen.

Where it turns up

Most atmospheric ozone sits in the stratosphere, from about 10 to 50 kilometres above the surface. Sunlight’s ultraviolet makes it there from oxygen. Brought down to ordinary pressure and temperature, Los Alamos says, all the ozone in the atmosphere would make a layer 3 mm thick.

Closer to the ground it is the main ingredient of smog. Nothing emits it directly: the US Environmental Protection Agency (EPA) says it forms when oxides of nitrogen and volatile organic compounds from cars, power plants and refineries react in sunlight. Levels tend to peak on hot, sunny days in cities. Wind carries it far enough to raise rural levels too.

Smaller sources are closer to hand. Sparking electrical equipment owes its sharp smell partly to ozone, and photocopiers make it as well. Arc welding produces it, most of all on aluminium, and white blood cells in the body produce it too.

Why it matters

High up, ozone blocks ultraviolet light by absorbing it. Each absorption breaks a molecule into an oxygen atom and O₂, and the atom rejoins dioxygen to rebuild ozone. OpenStax links that shield to a lower frequency of skin cancer on the ground.

In 1974 Mario Molina and F. Sherwood Rowland warned in Nature that chlorofluorocarbons (CFCs) threatened the layer. Ultraviolet light, or reaction with hydroxyl radicals, frees chlorine atoms from CFCs. Each chlorine atom destroys ozone and is regenerated, so one can break down thousands of molecules. Most atmospheric chlorine, though, sits in inactive forms, Cl₂ and ClONO₂, and nitric oxide is another catalyst of the same breakdown. The warning helped bring about the Montreal Protocol of 1987, and the Antarctic hole is now shrinking.

Too costly to ship, ozone is usually made where industry uses it. It bleaches oils, waxes, fabrics and starch by oxidising their coloured compounds to colourless ones, and is also used to bleach flour and paper pulp. Its reaction with unsaturated fatty acids yields pelargonic and azelaic acids.

Properties

Properties of ozone Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 24823.
Property	Value	Unit	Source
Molecular formula (Hill order)	O₃
Molar mass	47.9970(30) g/mol	g/mol	IUPAC/CIAAW 2024
Oxygen atoms	3
Bonds	2
Single bonds	1
Double bonds	1
PubChem molecular weight	47.998 g/mol	g/mol	PubChem

Safety

Ground-level ozone is a harmful air pollutant. In companies’ notifications to the European Chemicals Agency (ECHA), the concentrated gas carries the signal word Danger, as an oxidiser that is fatal if inhaled. Breathing it can cause fluid to build up in the lungs, with effects that may be delayed. The US Environmental Protection Agency names children, older people and people of any age with lung diseases such as asthma as the most affected. The authoritative references are the PubChem Laboratory Chemical Safety Summary and the NIOSH (US National Institute for Occupational Safety and Health) card for ozone.

Questions

What shape is an ozone molecule?

Bent. OpenStax describes ozone as a bent molecule, and the 3D model shows the middle oxygen holding its two neighbours at an angle rather than in a straight line. The model draws one bond single and the other double, but that is just one of ozone's resonance structures. In OpenStax's general account of resonance, the real electrons are spread as an average of the possible forms.

Why is ozone good in the stratosphere but bad at ground level?

It is the same gas in two places. High up, the US Environmental Protection Agency calls it good, since by absorbing ultraviolet radiation it shields plants, animals and people. Near the ground the agency calls it bad, because breathing it can set off a range of health problems. Street-level ozone forms when oxides of nitrogen and volatile organic compounds react in sunlight, and it is the main ingredient of smog.

What does ozone smell like?

The NIOSH Pocket Guide describes a very pungent odour, and the sharp smell around sparking electrical equipment comes partly from ozone, according to OpenStax. Lightning, photocopiers and ultraviolet light all generate it. The nose picks it up at very low levels: PubChem's experimental data give an odour threshold starting at 0.0076 parts per million.

Is ozone used to treat drinking water?

Yes, as an alternative to chlorine. It disinfects water and air by virtue of its oxidising power, and PubChem's use data add taste and odour control. It is usually made on the spot, since the gas is too expensive to ship. The same data carry a caution: under certain conditions ozonation can produce by-products that are mutagenic and possibly carcinogenic, meaning able to damage genes or cause cancer.

Same shape (bent)

H₂O Water 3 atoms, 2 single bonds, bent.