Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Radon, symbol Rn, colorless, odorless radioactive gaseous element that is the heaviest of the noble gases of the periodic table. The atomic number of radon is 86.

Radon-222 was discovered in 1900 by the German chemist Friedrich Ernst Dorn. Radon was believed to be chemically inert. Since 1962, however, chemists have been able to make radon compounds. Radon-222, which is the most abundant isotope of radon, is formed by the radioactive decay of radium-226 (see Radioactivity). Radon-222 has a half-life of 3.8 days, decaying by the emission of alpha particles into an isotope of the element polonium. Radon makes up most normal background radioactivity. This colorless gas forms from the decay of uranium and radium, elements that are naturally present in rock and soil. Radon can concentrate in basements and other unventilated indoor areas built into the soil. Indoor accumulations of radon pose a serious health hazard. When radon breaks down, it produces charged particles that adhere to dust and other fine matter that can be inhaled by people. Studies indicate that radon may cause up to 21,800 deaths from lung cancer each year in the United States. Several states in the northeastern United States have developed programs to determine whether the gas is present in amounts high enough to pose a risk of lung cancer.

Radon-222 is obtained by passing air through a solution of radium salt and collecting the air and the radon gas that was present in the solution. This isotope can be used in the treatment of malignant tumors. The gas is enclosed in a tube, usually made of glass or gold, called a radon seed, which is inserted in the diseased tissue.

Nineteen other isotopes of radon are known. The isotope of mass 220, discovered in 1899 by Ernest Rutherford, is a product of the radioactive decay of an isotope of thorium and is known as thoron; it has a half-life of 55 seconds. The isotope of mass 219, with a half-life of 4 seconds, is a product of the radioactive decay of an isotope of actinium and is known as actinon. Radon melts at about -71° C (about -96° F), boils at -62° C (-80° F), and has a density of 9.73 g/liter at 0° C (32° F) and 1 atmosphere pressure.