Science & Technology in Action

15th Edition

Ozone and Ozone-depleting substances


Low energy ultraviolet can break ozone molecules apart. However, certain chemical species can break not just one ozone molecule but maybe 100,000 of them before they are themselves removed from the atmosphere. They act like catalysts for the destruction of ozone. The principal ozone destroyers are: chlorine and bromine atoms, hydroxyl radicals (OH•) and nitric oxide (NO)

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The stratospheric ozone layer acts as an invisible shield and protects us from harmful ultraviolet (UV) radiation from the sun. Long-term exposure to high levels of UV-B can severely damage most animals, plants and microbes, so the ozone layer protects all life on Earth. The Earth’s atmosphere is 78% nitrogen (N2) and 21% oxygen (O2). It also contains reactive molecules and free radicals which provide most of the oxidising power of the atmosphere. These are formed almost entirely by photochemistry: ozone (O3), peroxy radicals (HO2•, RO2•), hydroxyl radicals (OH•) and peroxides (e.g. H2O2 and RO2H). Note: The letter ‘R’ in these formulas usually stands for an organic ‘radical’ such as CH3- C2H5- etc. The dot in formulas such as OH• indicates the presence of an unpaired electron. Such species are known as free radicals and are generally very reactive.

What is ozone?

Ozone is an allotrope of oxygen which has three oxygen atoms in each molecule. It has a distinctive smell reminiscent of chlorine. Like chlorine it is chemically reactive and toxic and can be used as a disinfectant. Ozone in the stratosphere protects us from most of the harmful solar UV radiation. About 10% of atmospheric ozone occurs in the troposphere, i.e. the lowest layer of the atmosphere, typically in concentrations of around 20 or 30 ppm (parts per million). Higher concentrations (e.g. 100 ppm) occur in heavily polluted areas. Ozone in the troposphere is undesirable because it increases incidence of respiratory diseases.

True or False?

  1. Nitrogen is the most abundant gas in the atmosphere. true
  2. Molecules containing unpaired electrons are know as free radicals and are chemically very reactive. true
  3. The lowest layer of the atmosphere is the stratosphere. false
  4. Ozone is formed by different mechanisms in different parts of the atmosphere. true
  5. Stratospheric ozone absorbs harmful solar UV rays. true
  6. Chlorine and bromine atoms act as catalysts for the destruction of stratospheric ozone. true
  7. Under normal conditions CFCs (chloro-fluoro-carbons) are chemically very reactive. false
  8. Hydroxyl radicals are the main oxidising species in the lower atmosphere. true
  9. Nitrogen dioxide, carbon monoxide and volatile organic compounds are ozone precursors. true

Glossary of terms

different forms of an element; graphite and diamond are allotropes of carbon
substance that changes the rate of reaction but is not used by the reaction itself
the transfer (of heat) within fluids (liquids and gases) by bulk movement of the fluid arising from density changes caused by local heating; the hot fluid generally rises
a substance that kills pathogens
free radical
or simply 'radical'; an atom, molecule or ion with unpaired electrons in the outer shell
the number of complete oscillations or cycles per second
hydroxyl radical
a very reactive compound of hydrogen and oxygen; it is not electrically charges (unlike OH- ions)
maximum wavelength
the longest wavelength in a range and therefore the lowest frequency and lowest energy
nitrogen dioxide
NO2, one of several oxides of nitrogen; it is a brown coloured gas with a pungent smell; liquefies below 21°C
namometre; one millionth of a millimetre; 10−9 m
oxidising power
ability to remove electrons form atoms or molecules
ozone precursor
it leads to the formation of ozone
chemical reaction that are initiated by light, especially UV light
breaking of chemical bonds by the action of light
the unit of electromagnetic energy; single quantum of radiant energy.
Planck’s constant
the constant of proportionality between the frequency of an electromagnetic wave and it energy (6.626 × 10^−34 J s)
parts per million
unpaired electron
Electron orbitals can hold just two electrons; an unpaired electron is on its own in an orbital
ultraviolet; beyond the violet end of the visible spectrum
volatile organic compounds