Rare Earth Elements

What are Rare Earth Elements?

Rare earth elements, also called lanthanides or inner transition metals are the chemical elements from cerium (Ce) to lutetium (Lu) or f-block elements of the periodic table.  Lanthanum is part of d-block elements according to their outer electronic configuration (5d¹ 6s²). The physical and chemical properties and common oxidation number or state are similar to that of the lanthanides series. Therefore, lanthanum is part of the lanthanides series.

The seventeen elements (fifteen lanthanides and two elements like scandium and yttrium) together are called rare earth elements.

Lanthanide Elements

All the elements from lanthanum to lutetium are members of the lanthanides series.

But the seventeen elements from lanthanum to lutetium, scandium, and yttrium are members of the rare earth family.

Why Lanthanides are Called Rare Earth Elements?

The name lanthanides is given rare earth elements 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 is avoided now because many of these elements are no longer rare but abundant.

Electronic Configuration of Rare Earth Metals

The general electronic configuration of lanthanides is 4f^{0, 2 to 14} 5d^0,1 6s². The elements scandium and yttrium are considered to be d-block elements. However, due to chemical properties and occurrence, these two elements are also members of the rare earth family.

Rare earth element electronic configuration
Elements	Symbol	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] 4f1 5d1 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

Properties of Lanthanides

• All the rare earth metals are silvery-white, soft, and malleable with high electrical conductance. Europium and ytterbium are a pale yellow colour.
• They are highly electropositive and reactive and the heavier elements are 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 elements burn in the air to form oxides and nitrides with oxygen and nitrogen.

Oxidation State of Rare Earth Elements

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 is equal to its oxidation state or number.

Magnetic Properties of Rare Earth Elements

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.

What is Lanthanide Contraction?

Except for scandium and yttrium, the atomic and ionic radii of rare earth metals steadily decrease along with the lanthanide series. This is commonly known as lanthanide construction.

Poor shielding electrons of 4f-orbitals 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.

Where are Rare Earth Metals Found?

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 heavier rare earth elements are virtually absent from it.
• Xenotime is also another rare-earth orthophosphate containing thorium and a high percentage of yttrium.

Monazite is a chemically quite inert rare earth mineral. It is found in high density in beach sands and river beds through weathering. The rare earth mineral monazite is found mainly in India, China, South Africa, Australia, and Malaysia.

Why are Rare Earth Metals Important?

All rare earth metals are very important elements because they 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, zirconium, iron, cobalt, nickel, manganese, tungsten, uranium, boron, and silicon.

What is Mischmetal?

Alloys of lanthanides are known as mischmetals. The rare earth element, cerium (45 to 50 percent), lanthanum (25 percent), and neodymium (5 percent) are 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 Elements

The uses of rare-earth compounds can be broadly classified into two types, non-nuclear applications, and nuclear applications.

Rare Earth Oxides Ceramics

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 coming from nuclear reactions.

The oxides of rare earth elements absorbed ultraviolet rays from sunlight. Therefore, the glasses that contain lanthanum oxide are used for the production of sunglasses, and goggles for glass blowing, and welding work.

Paints and 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 textile industries, we used ceric salts for drying clothes. Chloride and acetate of rare earth elements are used for making fabrics that are water-proof and acid-resistant.

Catalytic 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.

Nuclear Applications

Lanthanide 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 are used in solid oxide fuel cells.