Rather than referring to their natural abundance, the word reflects their property of "hiding" behind each other in minerals. Despite their abundance, the technical term "lanthanides" is interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek λανθανειν ( lanthanein), "to lie hidden". The term "lanthanide" was introduced by Victor Goldschmidt in 1925. 3.4 Coordination chemistry and catalysis.However, owing to wide current use, "lanthanide" is still allowed. The 1985 IUPAC "Red Book" (p. 45) recommends using "lanthanoid" instead of "lanthanide", as the ending "-ide" normally indicates a negative ion. This convention is entirely a matter of aesthetics and formatting practicality a rarely used wide-formatted periodic table inserts the 4f and 5f series in their proper places, as parts of the table's sixth and seventh rows (periods). In presentations of the periodic table, the f-block elements are customarily shown as two additional rows below the main body of the table. Since "lanthanide" means "like lanthanum", it has been argued that lanthanum cannot logically be a lanthanide, but the International Union of Pure and Applied Chemistry (IUPAC) acknowledges its inclusion based on common usage. These elements are called lanthanides because the elements in the series are chemically similar to lanthanum. All lanthanide elements form trivalent cations, Ln 3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium. There is some dispute on whether lanthanum or lutetium is a d-block element, but lutetium is usually considered so by those who study the matter it is included due to its chemical similarities with the other 14. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide. These elements, along with the chemically similar elements scandium and yttrium, are often collectively known as the rare-earth elements or rare-earth metals. The lanthanide ( / ˈ l æ n θ ə n aɪ d/) or lanthanoid ( / ˈ l æ n θ ə n ɔɪ d/) series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium.
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