What is meant by rare earth elements?
Rare-earth element or lanthanides or inner transition elements are the chemical elements from cerium (Ce) to lutetium (Lu) or 4f-block elements of the periodic table. Lanthanum is part of d-block elements due to their outer electronic configuration (5d16s2) but they show a single oxidation number or state +3 and occur in the same ore deposits as the lanthanides. Therefore, lanthanum is the part of lanthanides. The seventeen elements (fifteen lanthanides and two elements like scandium and yttrium) together are called rare earth elements.
According to the definition, all the elements from lanthanum to lutetium is the member of lanthanides series. But the seventeen elements from lanthanum to lutetium, scandium, and yttrium are the member of the rare earth family.
The name rare earth was given to them because they are extracted from oxides for which the ancient name is earth. The elemental oxides of these metals are considered to be rare. The term rare earth elements are avoided now because many of these elements are no longer rare but abundant.
All the rare earth metals are silvery-white, soft, and malleable with high electrical conductance. Europium and ytterbium are pale yellow colour. They are highly electropositive and reactive and the heavier elements comparable to calcium and aluminum. The compact metals are quite stable to dry air at ordinary temperatures. In a humid atmosphere, they turn rapidly into dull. All of the rare earth element burn in the air to form oxides and nitrides with oxygen and nitrogen.
The general electronic configuration of lanthanides are 4f0, 2 to 14 5d0,1 6s2. The elements scandium and yttrium are considered to d-block elements. Due to chemical properties and occurrence, these two elements also the member of rare earth family.
|Rare-earth element electronic configuration|
|Rare-earth element||Atomic number||Electronic configuration|
|Scandium (Sc)||21||[Ar] 3d1 4s2|
|Yttrium (Y)||39||[Kr] 4d1 5s2|
|Lanthanum (La)||57||[Xe] 4f0 5d1 6s2|
|Cerium (Ce)||58||[Xe] 4f2 5d0 6s2|
|Praseodymium (Pr)||59||[Xe] 4f3 5d0 6s2|
|Neodymium (Nd)||60||[Xe] 4f4 5d0 6s2|
|Promethium (Pm)||61||[Xe] 4f5 5d0 6s2|
|Samarium (Sm)||62||[Xe] 4f6 5d0 6s2|
|Europium (Eu)||63||[Xe] 4f7 5d0 6s2|
|Gadolinium (Gd)||64||[Xe] 4f7 5d1 6s2|
|Terbium (Tb)||65||[Xe] 4f9 5d0 6s2|
|Dysprosium (Dy)||66||[Xe] 4f10 5d0 6s2|
|Holmium (Ho)||67||[Xe] 4f11 5d0 6s2|
|Erbium (Er)||68||[Xe] 4f12 5d0 6s2|
|Thulium (Tm)||69||[Xe] 4f13 5d0 6s2|
|Ytterbium (Yb)||70||[Xe] 4f14 5d0 6s2|
|Lutetium (Lu)||71||[Xe] 4f14 5d1 6s2|
The stable and common oxidation state of rare earth metals is +3 (III). It is possible to correlate the stability of lanthanides in various oxidation states with the electronic configuration of their ions. On the basis of the general rule, half-filled and completely filled 4f-orbitals are highly stable. Therefore, Ce+4, La+3 (4f0), Tb+4, Eu+2, Gd+2 (4f7) and Yb+2, Lu+3 (4f14) ions are stable. The charge of the respective ions equal to its oxidation state or number.
The paramagnetic property of an ion or an atom defines the presence of an unpaired electron on it. Since both the rare-earth ions like La+3 (4f0 5d0 6s0) and Lu+3 (4f14 5d0 6s0) have no unpaired electrons. Therefore, these two rare-earth ions have diamagnetic properties. Other ions rare-earth elements like Ln+3 (Ln = lanthanides) show paramagnetic properties.
Atomic and ionic radii
Except for scandium and yttrium, the atomic and ionic radii of rare earth element steadily decreases along with the lanthanide series. This is commonly known as lanthanide construction. Poor shielding electron of 4f-orbitals causes to steadily raise the effective nuclear change along with the series. Due to this fact, the atomic and ionic radii of lanthanides decrease across the series.
There are more than 200 minerals known which contain rare-earth elements. The two most commercially important minerals of rare earth are monazite and bastnaesite. Monazite is a mixed phosphate of lanthanum, cerium, thorium, and other rare earth metals. While bastnaesite is a fluoride carbonate of lanthanides and other rare earth metals but heavier rare earth elements are virtually absent on it. Xenotime is also another rare-earth orthophosphate containing thorium and a high percentage of yttrium.
Monazite, which is chemically quite inert rare earth minerals. It contains high density in beach sands and river beds through weathering. India, China, South Africa, Australia, and Malaysia are the main source of monazite minerals.
Uses of rare earth element
All the rare earth elements are used widely in different chemical processes and chemical industries. Lanthanides or rare-earth metals are used in metallothermic reactions due to their extraordinary reduction character. The process used for the production of pure niobium (Nb), zirconium (Zr), iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), tungsten (W), uranium (U), boron (B), and silicon (Si).
Uses in mischmetals
Alloys of lanthanides are known as mischmetals. The rare earth element, cerium (45 to 50 percent), lanthanum (25 percent), neodymium (5 percent) is the major constituents of mischmetals. It is used for the production of different brands or types of steel with high corrosion resistance and workability. It is an excellent scavenger for the adsorption of elements like oxygen and sulfur. Rare earth alloys are used for making permanent magnets. Neodymium is particularly used in making magnetic alloys.
Uses of rare-earth compounds
The uses of rare-earth compounds can be broadly classified into two types like non-nuclear applications and nuclear applications.
For decolorizing glasses, we used oxides of rare earth elements like cerium, lanthanum, neodymium, and praseodymium. These elements are used for the production of protective transparent glass blocks. The blocks are used in nuclear technology to protect against radioactive radiation come from nuclear reactions. The oxides of rare earth elements absorbed ultraviolet rays from sunlight. Therefore, the gasses which contain lanthanum oxide used for the production of sunglasses, goggles for glass blowing, and welding work.
Paints, textiles industries
The rare earth compounds are used for the production of lakes, dyes, and paints for porcelain. Ceric salts are used for dying In textiles industries, we used ceric salts for drying the clothes. Chloride and acetate of rare earth elements are used for making fabrics that are water-proof and acid-resistant.
Catalytic and Nuclear applications
Certain compounds of lanthanides are used as a chemical catalyst for hydrogenation, dehydrogenation, and oxidation of various organic compounds. Cerium phosphate is a good chemical catalyst used for the production of petroleum.
Lanthanides elements and their compounds are important materials in the nuclear power generation process. For example, rare earth elements like gadolinium, samarium, europium, and dysprosium have large cross-sections for neutron capture reactions and are used in control rods of atomic piles. Radioactive isotopes of praseodymium used in solid oxide fuel cells.