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.

Rare Earth Elements

Rare Earth Elements, series of chemical elements of the periodic table. The rare earth elements (or rare earth metals) include the elements with atomic numbers 57 through 71, and, named in order, are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Yttrium (atomic no. 39) and scandium (atomic no. 21) are sometimes included in the group of rare earth elements. The elements cerium (atomic no. 58) through lutetium (atomic no. 71) are commonly known as the lanthanide series.


Chemically, most of the rare earth elements are trivalent (see Valence). Cerium alone forms compounds with a valence of +4. Most rare earth compounds are strongly paramagnetic. An alloy composed of iron and of rare earth metals, called misch metal, is pyrophoric. When scratched it gives off sparks capable of igniting flammable gases. It is used in cigarette lighters, miners' safety lamps, and automatic gas-lighting devices. Misch metal is also used in making aluminum and magnesium alloys and some types of steel. Europium is used as a phosphor activator. In a color television tube it provides the red color seen by the viewer.


Rare earth elements are separated from other elements in a mineral by precipitation with a suitable reagent. Separation of the rare earth elements from each other by ordinary chemical means is difficult because their chemical properties are similar, and the isolation of an individual element may involve hundreds of fractional crystallizations. With the use of ion-exchange methods the separation of an individual rare earth element can be accomplished with greater ease and precision.

Oxides of the rare earth elements are called rare earths, and are found in minerals that are actually more abundant than those of some other metals, such as those in the platinum group. The principal source of rare earths is the mineral monazite. Some other rare minerals that also contain small amounts of rare earths include cerite, gadolinite, and samarskite. See Chemical Elements .