What are Rare Earth Elements?
Rare earth elements are a set of seventeen indistinguishable silvery-white and luscious soft heavy metals. These are also commonly known as rare earth metals or rare earth oxides. These rare earth metals are spread across the periodic table as lanthanides. Scandium and Yttrium are also considered as part of these rare earth metals as they are found in the same ore deposits as lanthanides and exhibit similar chemical properties. But the magnetic and electronic properties of Scandium and Yttrium differ from lanthanides or rare earth metals.
Rare Earth Elements in Periodic Table
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Rare earth elements, also commonly known as lanthanides are basically the elements present in the top extended row placed below the main body of the periodic table and the rare earth metals stretched from Cerium (Ce) to Lutetium (Lu). The other rare earth metals are situated in the third row as twenty-first and thirty-ninth metals. These are Scandium and Yttrium. They exhibit the same chemical properties as other lanthanides. The rare earth metals are generally trivalent in nature but sometimes fewer lanthanides show different valencies. For example, Cerium, Praseodymium and Terbium are tetravalent. On the other hand, samarium, europium and ytterbium are divalent in nature. All these rare earth materials are so similar in their chemical properties that 25% of their uses are the same but 75% of their usage is based on their unique properties.
Properties of Rare Earth Metals
Though rare earth materials exhibit similar properties chemically but are very different in physical aspects and possess different electronic and magnetic properties. For example, Lanthanum being the prototype of the lanthanide series exhibits a melting point of 9180C or 16840F which is much lower than the melting point of Lutetium which is the last rare earth element of the series. The melting point of lutetium is observed to be 16630C or 30250F. Such wide differences of physical properties are not found in other groups of elements. For instance, the melting point of copper, silver and gold vary by merely 1000C. Thus few of the common properties of rare earth metals are as follows:
The metals are either silver, silvery-white or gray in colour.
They have high luster but tarnish easily when they come in contact with the air.
The metals have high electrical conductivity.
They share many common properties with each other and therefore it gets very difficult to separate or distinguish them.
The rare earth metals naturally occur together in minerals. For example, monazite is a mixed rare earth phosphate.
There is very little difference in the solubility and complex formation ability of the rare earth metals.
Rare earth metals are found with non-metals in their third oxidation state (3+). There is very little tendency of these metals to exhibit different oxidation states. An exception to this is Europium a with valency (2+) and Cerium with a valency of 4+.
Occurrence and Abundance of Rare Earth Elements
Though the abundance of the rare earth metals is very high, their availability is very low as their percentage of concentrations in ores are extremely low, as low as 5% availability by weight. Among all 83 elements that occur naturally these 16 rare earth elements fall in the 50 percentile of natural occurrence. Among them, Promethium, a radioactive element with the most stable isotope and a half-life of 17.7 years is not considered to be naturally occurring, although some traces of the element was found in ores.
Among all the naturally occurring rare earth elements, Cesium is the most abundant with the 28th ranking and Thulium, the least abundant with the 83rd ranking. It has been observed over the years that lighter lanthanides are in more abundance than heavy lanthanides. Though rare earth elements are distributed all over the world, the major concentrations are found in China, America, Australia and Russia. The other viable ores are found in Canada, India, South Africa and Southeast Asia.
The major rare earth materials found in these ores are bastnasite (fluorocarbonate), monazite (phosphate), loparite [(R, Na, Sr, Ca)(Ti, Nb, Ta, Fe3+) O3] and laterite clay (SiO2, Al2O3 and Fe2O3). As it was estimated in 2017, the total rare earth reserves are about 120 million metric tons. Most of these reserves are situated in China with about forty-four million metric tons of rare earth metal ores. America holds about 1.5 million metric tons of rare earth reserves. After China a decent amount of reserves are found in Russia, Brazil followed by Vietnam. Many of these countries use these rare earth metals for many key technologies in the medical and energy division. China and America use these metals for the manufacturing of lasers, battery electrodes, MRI contact agents, magnets, catalysts, alloys etc.
FAQs on Rare Earth Elements
1. What are rare earth elements (REEs) and what are they also known as?
Rare earth elements (REEs) are a group of seventeen metallic elements that exhibit similar chemical properties. They are not actually as rare as their name suggests. This group includes the 15 lanthanides from the periodic table, plus scandium (Sc) and yttrium (Y), which are found in the same ore deposits and share similar characteristics. They are also commonly known as rare earth metals or lanthanoids (when referring to the f-block series).
2. Where are the rare earth elements located in the periodic table?
The rare earth elements occupy a specific section of the periodic table.
- The 15 lanthanides (from Lanthanum, atomic number 57, to Lutetium, 71) are located in the f-block, which is usually shown as a separate two-row block at the bottom of the main table.
- Scandium (Sc) and Yttrium (Y) are transition metals found in Group 3 of the d-block. They are included as REEs due to their chemical similarities and natural co-occurrence with the lanthanides.
3. Why are the rare earth elements called 'rare'?
The term 'rare' is a historical misnomer and can be misleading. These elements are called rare not because they are scarce in the Earth's crust (in fact, cerium is more abundant than copper), but because they are typically found dispersed and not in concentrated, economically exploitable ore deposits. Their chemical similarity makes them very difficult and costly to separate from one another, contributing to the perception of their 'rarity'.
4. What are the 17 rare earth elements?
The 17 rare earth elements are comprised of yttrium, scandium, and the 15 lanthanide elements. The complete list is:
- 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)
- Lutetium (Lu)
5. What are some important uses of rare earth elements in modern technology?
Rare earth elements are crucial components in a wide range of modern technologies due to their unique magnetic, phosphorescent, and catalytic properties. Key applications include:
- Permanent Magnets: Neodymium and samarium are used to make powerful, lightweight magnets for motors in electric vehicles, wind turbines, hard drives, and headphones.
- Electronics: They are used in polishing glass for smartphone screens, as phosphors in displays (like Europium for red colour), and in fibre optic cables.
- Catalysts: Cerium is used in catalytic converters in cars to reduce emissions and in the petroleum refining process.
- Medical Technology: Gadolinium is used as a contrast agent for MRI scans.
6. Is gold a rare earth element?
No, gold (Au) is not a rare earth element. Gold is classified as a precious metal and a transition metal, located in Group 11 of the periodic table. While it is rare and valuable, it does not share the specific chemical and physical properties of the 17 rare earth elements, such as their electron configuration (f-orbital electrons) and their tendency to form 3+ ions.
7. Why are the rare earth elements so chemically similar to one another?
The remarkable chemical similarity among the lanthanoids is due to their unique electronic configuration. As you move across the lanthanide series, the additional electron enters the inner (n-2)f subshell, specifically the 4f subshell. The outermost shell, which primarily determines chemical behaviour, contains the same number of electrons (two 6s electrons). This means they all tend to form a stable +3 oxidation state, resulting in very similar chemical properties and making their separation a significant challenge.
8. What is lanthanoid contraction and what are its consequences?
Lanthanoid contraction is the steady decrease in the atomic and ionic radii of the lanthanide elements as the atomic number increases. This occurs because the electrons being added to the inner 4f subshell provide a poor shielding effect. As the nuclear charge increases, the outer electrons are pulled more strongly towards the nucleus, causing the atom to shrink.
Consequences: The most significant consequence is that the atomic radii of the elements following the lanthanides (in the 3rd transition series, e.g., Hafnium) are almost identical to the elements directly above them (in the 2nd transition series, e.g., Zirconium). This similarity in size leads to very similar chemical properties for pairs like Zr/Hf.
9. Does India have significant reserves of rare earth elements?
Yes, India has the fifth-largest reserves of rare earth elements in the world, as per official surveys. The primary mineral source for REEs in India is monazite sand, which is found in beach placers along the coasts of several states, including Kerala, Tamil Nadu, and Odisha. Indian Rare Earths Limited (IREL) is the main government body responsible for the mining and processing of these strategic elements.
