ozone (ōˈzōn) [key], an allotropic form of the chemical element oxygen (see allotropy). Pure ozone is an unstable, faintly bluish gas with a characteristic fresh, penetrating odor. The gas has a density of 2.144 grams per liter at STP. Below its boiling point ( - 112°C) ozone is a dark blue liquid; below its melting point ( - 193°C) it is a blue-black crystalline solid. Ozone is triatomic oxygen, O3, and has a molecular weight of 47.9982 atomic mass units (amu). It is the most chemically active form of oxygen. It is formed in the ozone layer of the stratosphere by the action of solar ultraviolet light on oxygen. Although it is present in this layer only to an extent of about 10 parts per million, ozone is important because its formation prevents most ultraviolet and other high-energy radiation, which is harmful to life, from penetrating to the earth's surface. Ultraviolet light is absorbed when its strikes an ozone molecule; the molecule is split into atomic and diatomic oxygen: 03+ ultraviolet light →0+02. Later, in the presence of a catalyst, the atomic and diatomic oxygen reunite to form ozone. Some environmental scientists fear that certain man-made pollutants (e.g., nitric oxide, NO) may interfere with this delicate balance of reactions that maintains the ozone's concentration, possibly leading to a drastic depletion of stratospheric ozone. Ozone is also formed when an electric discharge passes through air; for example, it is formed by lightning and by some electric motors and generators. Ozone is produced commercially by passing dry air between two concentric-tube or plate electrodes connected to an alternating high voltage; this is called the silent electric discharge method. Ozone is used commercially as a disinfectant and decontaminant for air and water, and as a bleaching agent for waxes, oils, and other organic compounds. The major commercial use is in the production by ozonolysis of azelaic acid (used in making plastics); it is also used in the synthesis of cortisone and certain synthetic sex hormones. Ozonization, the reaction of ozone with the double or triple bonds of unsaturated organic molecules, is useful in determining the structure of organic compounds.