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• Words to Know
• Ozone layer depletion
• Ozone in the troposphere
• Long-term problem

Ozone is an allotrope (a physically or chemically different form of the same substance) of oxygen with the chemical formula O₃. This formula shows that each molecule of ozone consists of three atoms. By comparison, normal atmospheric oxygen—also known as dioxygen—consists of two atoms per molecule and has the chemical formula O₂.

Ozone is a bluish gas with a sharp odor that decomposes readily to produce dioxygen. Its normal boiling point is −112°C (−170°F), and its freezing point is −192°C (−314°F). It is more soluble in water than dioxygen and also much more reactive. Ozone occurs in the lower atmosphere in very low concentrations, but it is...