Rare earths are the lanthanides in the periodic table of chemical elements - 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). And the two elements closely related to the 15 elements of the lanthanide series, scandium (Sc) and yttrium (Y), a total of 17 elements, called Rare Earth elements (Rare Earth). Rare earth (RE or R) for short.

Rare earth elements were originally found in relatively rare minerals produced in Sweden, and "soil" was a substance that was insoluble in water according to the custom at that time, so it was called rare earth.

According to the atomic electron layer structure and physical and chemical properties of rare earth elements, as well as their co-occurrence in minerals and the characteristics that different ionic radii can produce different properties, 17 rare earth elements are usually divided into two groups.

Light rare earths (also known as the cerium group) include: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, and gadolinium.

Heavy rare earths (also known as yttrium group) include: terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium.

The cerium group or yttrium group is named because the rare earth mixture obtained from the separation of minerals is often named after the cerium or yttrium.

The main physical and chemical properties of rare earth elements

Rare earth elements are typical metallic elements. Their metal activity is second only to alkali metals and alkaline earth metal elements, and more active than other metal elements. Among the 17 rare earth elements, arranged in the active order of metals, scandium, yttrium and lanthanum increase, and lanthanum decreases from lanthanum to lutetium, that is, lanthanum is the most active. Rare earth elements can form chemically stable oxides, halides and sulfides. Rare earth elements can react with nitrogen, hydrogen, carbon and phosphorus, and are easily soluble in hydrochloric acid, sulfuric acid and nitric acid.

Rare earths are easy to combine with oxygen, sulfur, lead and other elements to produce compounds with high melting points, so adding rare earths to molten steel can play a role in purifying steel. Because the metal atomic radius of rare earth elements is larger than that of iron, it is easy to fill in its grains and defects, and generate a film that can hinder the continued growth of grains, so as to refine the grains and improve the performance of steel.

Rare earth elements have an unfilled 4f electron shell structure, and thus produce a variety of electronic energy levels. Therefore, rare earth can be used as excellent fluorescent, laser and electric light source materials as well as colored glass, ceramic glaze.

Rare earth ions form complexes with hydroxyl, azo or sulfonic acid groups, making rare earth widely used in printing and dyeing industry. Some rare earth elements have a large neutron capture cross-sectional area, such as samarium, europium, gadolinium, dysprosium and erbium, which can be used as control materials and moderators in nuclear reactors. Cerium and yttrium have small neutron capture cross-sectional areas, which can be used as diluent for reactor fuel.

Rare earth has similar properties to trace elements, which can promote the seed germination of crops, promote root growth, and promote plant photosynthesis.