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.


Thorium, symbol Th, radioactive metallic element with an atomic number of 90. Thorium is a member of the actinide series of the periodic table.

Thorium was discovered (1828) by the Swedish chemist Baron Jöns Jakob Berzelius. The element is dark in color, slowly attacked by water, soluble in hydrochloric and sulfuric acids, and slightly soluble in nitric acid. It ranks 39th in abundance among the elements in the crust of the earth. Thorium melts at about 1750° C (about 3182° F), boils at about 4850° C (about 8762° F), and has a specific gravity of 11.8. The atomic weight of thorium is 232.04.

Small quantities of thorium are found in thorite, or thorium silicate; in orangite, a variety of thorite; and in thorianite, a radioactive mineral composed of thorium oxide and uranium.

Thorium has isotopes ranging in mass number from 212 to 236. Thorium-232 occurs naturally, has a half-life of about 14 billion years, and is the first member of the radioactive-decay series, ending with the stable lead isotope lead-208 (see Radioactivity). Thorium is currently important as a potential atomic-fuel source, because bombardment of thorium-232 by slow neutrons yields the fissile isotope uranium-233. This process is comparable to the process by which fast neutrons “breed” fissile plutonium-239 from nonfissile uranium-238 (see Nuclear Energy). The thorium-uranium fuel cycle is being studied by scientists as an alternative to the uranium-plutonium fuel cycle. Two types of reactors, the molten-salt breeder reactor and the light-water breeder reactor, are being considered. These programs may make available for power production the United States thorium-fuel reserves, which represent a source of energy many times greater than the known coal, oil, and natural-gas reserves of the country. Thorium metal is used in magnesium alloys and as a stabilizing component of electronic tubes. Thorium oxide is used in light filaments and electrodes and also as a catalyst.