Ozone << OH zohn >> is a gas that is present in small amounts in Earth’s atmosphere. In the troposphere (the lowest level of the atmosphere), ozone is a pollutant. It can harm plant and animal tissues, and it can damage rubber and plastic. Ozone in the stratosphere (the layer above the troposphere) blocks harmful ultraviolet rays from the sun, protecting life on Earth. Every spring in the Southern Hemisphere since the late 1970’s, scientists have observed a depletion of the ozone layer over Antarctica. The region of decreased ozone is known as the ozone hole. See Ozone hole.
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Ozone is related to the oxygen molecules that sustain life. An oxygen molecule has two oxygen atoms and the chemical formula O2. An ozone molecule has three oxygen atoms and the formula O3. Pure ozone is a pale blue gas. The word ozone comes from a Greek word meaning to smell, reflecting ozone’s sharp, irritating odor.
Ozone occurs naturally through photochemical reactions and by electrical discharges. In photochemical reactions in the stratosphere, ultraviolet rays from the sun strike oxygen molecules, breaking each molecule into two oxygen atoms. The oxygen atoms combine with other oxygen molecules, forming ozone. Electrical discharges include lightning and sparks from motors. Such discharges can break up oxygen molecules, leading to ozone formation. Near Earth’s surface, reactions between such gases as nitrogen oxides and hydrocarbons create ozone and other ingredients of a dangerous pollutant called photochemical smog.
Commercial applications of ozone include the bleaching of pulp in paper mills and the purification of water. Manufacturers produce ozone by creating electrical discharges in a machine.
Ozone’s relative molecular mass (formerly called molecular weight)—that is, its amount of matter compared with that of the most common form of carbon—is 47.998. The German chemist Christian Friedrich Schonbein discovered ozone in 1840.
