A lanthanide () (is any of the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. In the periodic table, the first fourteen (up to ytterbium) fill the 4f orbitals.Lanthanide , Encyclopædia Britannica on-line Lutetium (element 71) is also often considered a lanthanide, despite being a d-block element and a transition metal. The IUPAC lists the 15 elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, under the collective name lanthanoid (), which it recommends as chemically more correct for this series.
The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide. 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), 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.F Block Elements, Oxidation States, Lanthanides and Actinides . Chemistry.tutorvista.com. Retrieved on 14 December 2017. 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). Several lanthanides can also form +2 and even +4 oxidation states.
In presentations of the periodic table, the f-block elements are customarily shown as two additional rows below the main body of the table. 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.
Etymology The term "lanthanide" was introduced by Victor Goldschmidt in 1925.Skrifter Norske Vidensk-Akad. (Mat.-nat. Kl.) V:6 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"."lanthanide". OED2 1.14 on CD-Rom. OUP, 1994.
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,"lanthanum". OED2 v. 1.14 on CD-Rom. OUP, 1994. 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.
These elements are called lanthanides because the elements in the series are chemically similar to lanthanum. Because "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. The current IUPAC recommendation is that the name lanthanoid be used rather than lanthanide, as the suffix "-ide" is preferred for negative ions, whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. However, lanthanide is still commonly used.
Physical properties of the elements
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."The Elements", Theodore Gray, Black−Dog & Leventhal, Chicago 2007: "Lanthanum" and "Cerium" entries Ch 57 & 58, pp 134–7 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."The Elements", Theodore Gray, Black Dog & Leventhal, Chicago 2007: "Neodymium" and "Praseodymium" entries Ch 59 & 60, pp 138–43 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.
+ Some properties of the lanthanidesChemical elementLaCePrNdPmSmEuGdTbDyHoErTmYbLu Atomic number575859606162636465666768697071Image Density (g/cm3)6.1626.7706.777.017.267.525.2447.908.238.5408.799.0669.326.909.841Melting point (°C)9207959351024104210728261312135614071461152915458241652Boiling point (°C)346434433520307430001794152932733230256727202868195011963402 Atomic electron configuration (gas phase)*5d14f15d14f3 4f44f54f64f74f75d14f94f104f114f124f134f144f145d1Metal lattice (RT)dhcpfccdhcpdhcpdhcp**bcchcphcphcphcphcphcpfcchcpMetallic radius (pm)162181.8182.4181.4183.4180.4208.4180.4177.3178.1176.2176.1175.9193.3173.8Resistivity at 25 °C (μΩ·cm)57–8020 °C7368648890134114578787792979Magnetic susceptibilityχmol /10−6(cm3·mol−1)+95.9+2500 (β)+5530 (α)+5930 (α)+1278 (α)+30900+185000(350 K)+170000 (α)+98000+72900+48000+24700+67 (β)+183 * Between initial Xe and final 6s2 electronic shells
** Sm has a close packed structure like most of the lanthanides but has an unusual 9 layer repeat
Gschneider and Daane (1988) attribute the trend in melting point which increases across the series, (lanthanum (920 °C) – lutetium (1622 °C)) to the extent of hybridization of the 6s, 5d, and 4f orbitals. The hybridization is believed to be at its greatest for cerium, which has the lowest melting point of all, 795 °C.Krishnamurthy, Nagaiyar and Gupta, Chiranjib Kumar (2004) Extractive Metallurgy of Rare Earths, CRC Press, The lanthanide metals are soft; their hardness increases across the series. Europium stands out, as it has the lowest density in the series at 5.24 g/cm3 and the largest metallic radius in the series at 208.4 pm. It can be compared to barium, which has a metallic radius of 222 pm. It is believed that the metal contains the larger Eu2+ ion and that there are only two electrons in the conduction band. Ytterbium also has a large metallic radius, and a similar explanation is suggested. The resistivities of the lanthanide metals are relatively high, ranging from 29 to 134 μΩ·cm. These values can be compared to a good conductor such as aluminium, which has a resistivity of 2.655 μΩ·cm. With the exceptions of La, Yb, and Lu (which have no unpaired f electrons), the lanthanides are strongly paramagnetic, and this is reflected in their magnetic susceptibilities. Gadolinium becomes ferromagnetic at below 16 °C (Curie point). The other heavier lanthanides – terbium, dysprosium, holmium, erbium, thulium, and ytterbium – become ferromagnetic at much lower temperatures.Cullity, B. D. and Graham, C. D. (2011) Introduction to Magnetic Materials, John Wiley & Sons,
+ Electron configurations and colours of lanthanide ionsChemical elementLaCePrNdPmSmEuGdTbDyHoErTmYbLu Atomic number575859606162636465666768697071 Ln3+ electron configuration*4f0 4f14f2 4f3 4f44f5 4f6 4f74f8 4f9 4f104f114f12 4f134f14 Ln3+ radius (pm) 103 102 99 98.3 97 95.8 94.7 93.8 92.3 91.2 90.1 8988 86.8 86.1 Ln4+ ion color in aqueous solution — Orange-yellow Yellow Blue-violet — — — — Red-brown Orange-yellow — — — — — Ln3+ ion color in aqueous solution Colorless Colorless Green Violet Pink Pale yellow Colorless Colorless V. pale pink Pale yellow Yellow Rose Pale green Colorless Colorless Ln2+ ion color in aqueous solution — — — — — Blood red Colorless — — — — — Violet-red Yellow-green —
* Not including initial [Xe] core
f → f transitions are symmetry forbidden (or Laporte-forbidden), which is also true of transition metals. However, transition metals are able to use vibronic coupling to break this rule. The valence orbitals in lanthanides are almost entirely non-bonding and as such little effective vibronic coupling takes, hence the spectra from f → f transitions are much weaker and narrower than those from d → d transitions. In general this makes the colors of lanthanide complexes far fainter than those of transition metal complexes.
+Approximate colors of lanthanide ions in aqueous solutionHolleman, p. 1937.dtv-Atlas zur Chemie 1981, Vol. 1, p. 220. Oxidation state 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 +2 Sm2+ Eu2+ Tm2+ Yb2+ +3 La3+ Ce3+ Pr3+ Nd3+ Pm3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ +4 Ce4+ Pr4+ Nd4+ Tb4+ Dy4+
Effect of 4f orbitals Viewing the lanthanides from left to right in the periodic table, the seven 4f atomic orbitals become progressively more filled (see above and ). The electronic configuration of most neutral gas-phase lanthanide atoms is [Xe]6s24fn, where n is 56 less than the atomic number Z. Exceptions are La, Ce, Gd, and Lu, which have 4fn−15d1 (though even then 4fn is a low-lying excited state for La, Ce, and Gd; for Lu, the 4f shell is already full, and the fifteenth electron has no choice but to enter 5d). With the exception of lutetium, the 4f orbitals are chemically active in all lanthanides and produce profound differences between lanthanide chemistry and transition metal chemistry. The 4f orbitals penetrate the [Xe] core and are isolated, and thus they do not participate much in bonding. This explains why crystal field effects are small and why they do not form π bonds. As there are seven 4f orbitals, the number of unpaired electrons can be as high as 7, which gives rise to the large magnetic moments observed for lanthanide compounds.
Measuring the magnetic moment can be used to investigate the 4f electron configuration, and this is a useful tool in providing an insight into the chemical bonding. The lanthanide contraction, i.e. the reduction in size of the Ln3+ ion from La3+ (103 pm) to Lu3+ (86.1 pm), is often explained by the poor shielding of the 5s and 5p electrons by the 4f electrons.
Chemistry and compounds
The chemistry of the lanthanides is dominated by the +3 oxidation state, and in LnIII compounds the 6s electrons and (usually) one 4f electron are lost and the ions have the configuration [Xe]4f(n−1). All the lanthanide elements exhibit the oxidation state +3. In addition, Ce3+ can lose its single f electron to form Ce4+ with the stable electronic configuration of xenon. Also, Eu3+ can gain an electron to form Eu2+ with the f7 configuration that has the extra stability of a half-filled shell. Other than Ce(IV) and Eu(II), none of the lanthanides are stable in oxidation states other than +3 in aqueous solution.
In terms of reduction potentials, the Ln0/3+ couples are nearly the same for all lanthanides, ranging from −1.99 (for Eu) to −2.35 V (for Pr). Thus these metals are highly reducing, with reducing power similar to alkaline earth metals such as Mg (−2.36 V).
Lanthanide oxidation states