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The lanthanide (/ˈlænθənaɪd/) or lanthanoid (/ˈlænθənɔɪd/) series of chemical elements[a] comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals.[2][3][4] Lutetium (element 71) is also sometimes considered a lanthanide, despite being a d-block element and a transition metal.
The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide.[5] All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. Lutetium is a d-block element (thus also a transition metal),[6][7] and on this basis its inclusion has been questioned; however, like its congeners scandium and yttrium in group 3, it behaves similarly to the other 14. The term rare-earth element or rare-earth metal is often used to include the stable group 3 elements Sc, Y, and Lu in addition to the 4f elements.[8] All lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum (La) to lutetium (Lu).
These elements are called lanthanides because the elements in the series are chemically similar to lanthanum. 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.[1]
In presentations of the periodic table, the f-block elements are customarily shown as two additional rows below the main body of the table.[2] 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), respectively.
The 1985 IUPAC "Red Book" (p. 45) recommends using lanthanoid instead of lanthanide, as the ending -ide normally indicates a negative ion. However, owing to widespread current use, lanthanide is still allowed.
Etymology
The term "lanthanide" was introduced by Victor Goldschmidt in 1925.[9][10] 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".[11]
Rather than referring to their natural abundance, the word reflects their property of "hiding" behind each other in minerals. The term derives from lanthanum, first discovered in 1838, at that time a so-called new rare-earth element "lying hidden" or "escaping notice" in a cerium mineral,[12] and it is an irony that lanthanum was later identified as the first in an entire series of chemically similar elements and gave its name to the whole series.
Together with the stable elements of group 3, scandium, yttrium, and lutetium, the trivial name "rare earths" is sometimes used to describe the set of lanthanides. The "earth" in the name "rare earths" arises from the minerals from which they were isolated, which were uncommon oxide-type minerals. However, these elements are neither rare in abundance nor "earths" (an obsolete term for water-insoluble strongly basic oxides of electropositive metals incapable of being smelted into metal using late 18th century technology). Group 2 is known as the alkaline earth elements for much the same reason.
The "rare" in the name "rare earths" has more to do with the difficulty of separating of the individual elements than the scarcity of any of them. By way of the Greek dysprositos for "hard to get at", element 66, dysprosium was similarly named. The elements 57 (La) to 71 (Lu) are very similar chemically to one another and frequently occur together in nature. Often a mixture of three to all 15 of the lanthanides (along with yttrium as a 16th) occur in minerals, such as monazite and samarskite (for which samarium is named). These minerals can also contain group 3 elements, and actinides such as uranium and thorium.[13] A majority of the rare earths were discovered at the same mine in Ytterby, Sweden and four of them are named (yttrium, ytterbium, erbium, terbium) after the village and a fifth (holmium) after Stockholm; scandium is named after Scandinavia, thulium after the old name Thule, and the immediately-following group 4 element (number 72) hafnium is named for the Latin name of the city of Copenhagen.[13]
The properties of the lanthanides arise from the order in which the electron shells of these elements are filled—the outermost (6s) has the same configuration for all of them, and a deeper (4f) shell is progressively filled with electrons as the atomic number increases from 57 towards 71.[13] For many years, mixtures of more than one rare earth were considered to be single elements, such as neodymium and praseodymium being thought to be the single element didymium.[14] Very small differences in solubility are used in solvent and ion-exchange purification methods for these elements, which require repeated application to obtain a purified metal. The diverse applications of refined metals and their compounds can be attributed to the subtle and pronounced variations in their electronic, electrical, optical, and magnetic properties.[13]
By way of example of the term meaning "hidden" rather than "scarce", cerium is almost as abundant as copper;[13] on the other hand promethium, with no stable or long-lived isotopes, is truly rare.[15]
Physical properties of the elements
| Chemical element | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
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| Atomic number | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 |
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| Density (g/cm3) | 6.162 | 6.770 | 6.77 | 7.01 | 7.26 | 7.52 | 5.244 | 7.90 | 8.23 | 8.540 | 8.79 | 9.066 | 9.32 | 6.90 | 9.841 |
| Melting point (°C) | 920 | 795 | 935 | 1024 | 1042 | 1072 | 826 | 1312 | 1356 | 1407 | 1461 | 1529 | 1545 | 824 | 1652 |
| Boiling point (°C) | 3464 | 3443 | 3520 | 3074 | 3000 | 1794 | 1529 | 3273 | Zdroj:https://en.wikipedia.org?pojem=Lanthanides