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There are 17 different rare earth elements, including the 15 lanthanide and two transitional metals. The first of these elements were discovered in the late 18th century in the form of a mineral ore that contained two lanthanide in addition to silicon and iron. They were originally referred to as rare earths since it was uncommon to find them in economically exploitable mineral deposits, but rare earth elements are in fact not rare at all, and some of them are actually quite abundant. These elements are very important to modern technology, and they can be used in everything from cellular phones to televisions and batteries for electric cars.
The lanthanide are often lumped together with yttrium and scandium, which are transitional metals, due to a number of factors. These elements can often be found in the same mineral deposits, and they also share certain chemical similarities. Yttrium, for example, is never found as a free element. Instead, it is always located in mineral compounds with other elements, many of which are typically lanthanide. The physical properties of yttrium also allow it to fit between the atomic weights of gadolinium and erbium, and it has a similar chemical reactivity to two other lanthanide.
Rare earth elements are not especially scarce, so the term can be somewhat misleading. Certain rare earth elements have an abundance similar to copper, and even the most scarce is substantially more common than gold. The reason they are referred to as rare is the way they are found in nature. It is relatively uncommon to find rare earth elements in an economically exploitable mineral form.
The mining and processing of rare earth elements can be expensive and difficult. Since these elements are often widely dispersed, a great amount of material typically needs to be processed to obtain a relatively small percentage of rare earths. The processing can also result in radioactive waste materials, which may require extensive permitting and abatement procedures.
Many rare earth elements are instrumental to modern technology. Several are used to make lasers, magnets, and phosphors. Computer memory, cellular phones, televisions, and other devices can all make extensive use of these elements. Cerium is the most common rare earth element, and it is used to reduce carbon monoxide emissions in a number of ways. An oxide of cerium is often added to diesel fuel, and the element can also used in automobile exhaust systems to catalyze undesirable chemicals into less harmful states.