The rare earths are a moderately abundant group of 17 elements comprising the 15 lanthanides, scandium, and yttrium. The elements range in crustal abundance from cerium, the 25th most abundant element of the 78 common elements in the Earth’s crust at 60 parts per million (ppm), to thulium and lutetium, the least abundant rare-earth elements at about 0.5 ppm (Mason and Moore, 1982, p. 46). In rock-forming minerals, rare earths typically occur in compounds as trivalent cations in carbonates, oxides, phosphates, and silicates.
The lanthanides comprise a group of 15 elements with atomic numbers 57 through 71 that include the following in order of atomic number (atomic symbol):
lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).
The rare earths are often described as being a "light-group rare-earth element" (LREE) or "heavy-group rare-earth element" (HREE).
The definition of a LREE and HREE is based on the electron configuration of each rare-earth element.
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The LREE are defined as lanthanum, atomic number 57 through gadolinium, atomic number 64. This is based on the fact that starting with lanthanum, which has no 4f shell electrons, clockwise spinning electrons are added for each lanthanide through gadolinium. Gadolinium has seven clockwise spinning 4f electrons, which creates a very stable, half-filled electron shell. The LREE also have in common increasing unpaired electrons, from 0 to 7.
The HREE are defined as terbium, atomic number 65 through lutetium, atomic number 71, and also yttrium, atomic number 39. This is based on the fact that starting with terbium, counter-clockwise spinning electrons are added for each lanthanide through lutetium. All of the HREE therefore differ from the first eight lanthanides in that they have “paired” electrons (a clockwise and counter-clockwise spinning election). The LREE have no paired electrons. Yttrium is included in the HREE group based on its similar ionic radius and similar chemical properties. In its trivalent state, which is similar to the other REE, yttrium has an ionic radius of 90 picometers, while holmium has an ionic radius of 90.1 picometers.
Scandium is also trivalent; however, its other properties are not similar enough to classify it as either a LREE or HREE.
The differences in electronic configuration are critical to the individual properties the rare-earth elements exhibit and how they interact with other elements and compounds.
1. Scandium (Sc) is more abundant in the Moon than on Earth.
2. Lanthanum (La) carbonate is used to reduce blood levels of phosphate in patients with kidney disease.
3. Cerium (Ce) is widely used in automotive catalytic converters to reduce pollution.
4. The primary use of praseodymium (Pr) is to combine it with neodymium magnets to supply the growing demand for high-tech applications.
5. A thumbnail size, high-strength neodymium (Nd) magnet made with iron (Fe) and boron (B) is so strong that when placed on a refrigerator it cannot be removed by hand.
6. Promethium (Pm) is used as a starter switch in energy-efficient compact fluorescent lamps (CFL).
7. Samarium (Sm) chloride taken internally in the proper quantity will combine with alcohol and keep you from becoming drunk.
8. Europium (Eu) is used in anti-counterfeiting fluorescent phosphors in Euro banknotes.
9. Using gadolinium (Gd) alloy Gd5(Si2Ge2) in magnetic refrigeration has the potential to cut electrical heating and cooling costs by as much as 80%.
10. Terbium (Tb) in an alloy with dysprosium and iron metals known as Terfenol-D can expand and contract in a magnetic field to precisely aim lasers.
11. Dysprosium (Dy) is injected into joints in the body to treat rheumatoid arthritis.
12. Holmium (Ho) is one of the colorants used in cubic zirconia, for use in jewelry, providing the dichroic colors of peach and yellow.
13. In sunglasses, erbium (Er) oxide enhances color perception and improves both contrast and depth perception.
14. Thulium (Tm) lasers are used in open, laparoscopic, and endoscopic medical procedures to cut and cauterize.
15. Ytterbium (Yb) lasers are used to drill into diamonds to remove imperfections.
16. The radioisotope Lutetium (Lu) is used for radiation therapy of small, soft tumors.
17. Yttrium (Y) provides a high temperature corrosion resistance in cutting tools.