Rare earth elements originally existed in rare minerals produced in Sweden, and "soil" was called water-insoluble substance according to the custom at that time, so it was called rare earth.
According to the electronic shell structure and physical and chemical properties of rare earth elements, as well as their occurrence in minerals and the characteristics that different ionic radii can produce different properties, seventeen rare earth elements are usually divided into two groups.
Light rare earth elements (also known as cerium group) include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium and gadolinium.
Heavy rare earths (also known as yttrium family) include: terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium.
It is called cerium group or yttrium group because cerium or yttrium is often dominant in the rare earth mixture obtained by mineral separation.
Main physical and chemical properties of rare earth elements
Rare earth elements are typical metallic elements. Their metal activity is second only to alkali metal and alkaline earth metal elements, but more active than other metal elements. In 17 rare earth elements, scandium, yttrium and lanthanum increase and decrease from lanthanum to lutetium according to the active order of metals, that is, lanthanum is the most active element. 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 earth can easily combine with oxygen, sulfur, lead and other elements to form compounds with high melting point, so adding rare earth to molten steel can purify steel. Because the metal atomic radius of rare earth elements is larger than that of iron, it is easy to fill its grains and defects and form a film that can hinder the continuous growth of grains, thus refining the grains and improving the properties of steel.
Rare earth elements have unfilled 4f electron shell structure, which leads to various electronic energy levels. Therefore, rare earth can be used as excellent fluorescent, laser and electric light source materials and glaze of colored glass and ceramics.
Rare earth ions form a combination with hydroxyl, azo group or sulfonic group, which makes rare earth widely used in printing and dyeing industry. However, some rare earth elements, such as samarium, europium, gadolinium, dysprosium and erbium, have the characteristics of large neutron capture cross section and can be used as control materials and deceleration agents for atomic energy reactors. However, cerium and yttrium can be used as diluents for reactor fuel because of their small neutron capture cross-sectional area.
Rare earth has properties similar to trace elements, which can promote crop seed germination, root growth and plant photosynthesis.
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