Lanthanides

  The lanthanides are located in block 5//d// of the __ [|periodic table] __. The first 5//d// transition element is either lanthanum or lutetium, depending on how you interpret the __ [|periodic trends] __ of the elements. Sometimes only the lanthanides, and not the actinides, are classified as rare earths. The lanthanides are not as rare as was once thought; even the scarce rare earths (e.g., europium, lutetium) are more common than the platinum-group metals. Several of the lanthanides form during the fission of uranium and plutonium. T he lanthanides have many scientific and industrial uses. Their compounds are used as catalysts in the production of petroleum and synthetic products. Lanthanides are used in lamps, lasers, magnets, phosphors, motion picture projectors, and X-ray intensifying screens. A pyrophoric mixed rare-earth alloy called Mischmetall (50% Ce, 25% La, 25% other light lanthanides) or misch metal is combined with iron to make flints for cigarette lighters. The addition of <1% Mischmetall or lanthanide silicides improves the strength and workability of low alloy steels. 

Chemistry and compounds
Lanthanide oxides: clockwise from top center: praseodymium, cerium, lanthanum, neodymium, samarium and gadolinium. The electronic structure of the lanthanide elements, with [|minor exceptions] is [Xe]6s24fn. In their compounds, the 6s electrons are lost and the ions have the configuration [Xe]4fm. [|[10]] The chemistry of the lanthanides differs from [|main group elements] and [|transition metals] because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons. As a consequence of this, the chemistry of the elements is largely determined by their size, which decreases gradually from 102 [|pm] (La3+) with increasing atomic number to 86 pm (Lu3+), the so-called [|lanthanide contraction]. 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 which has the extra stability of a half-filled shell. [|Promethium] is effectively a [|man-made element] as all its isotopes are radioactive with half-lives shorter than 20 y. 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). [|[7]]

Magnetic and spectroscopic
All the trivalent lanthanide ions, except lutetium, have unpaired f electrons. However the magnetic moments [|deviate considerably from the spin-only values] because of strong [|spin-orbit coupling]. The maximum number of unpaired electrons is 7, in Gd3+, with a magnetic moment of 7.94 [|B.M.], but the largest magnetic moments, at 10.4–10.7 B.M., are exhibited by Dy3+and Ho3+. However, in Gd3+ all the electrons have parallel spin and this property is important for the use of gadolinium complexes as [|contrast reagent] in [|MRI] scans. A solution of 4% [|holmium oxide] in 10% [|perchloric acid], permanently fused into a quartz cuvette as a wavelength calibration standard [|Crystal field splitting] is rather small for the lanthanide ions and is less important than spin-orbit coupling in regard to energy levels. [|[15]] Transitions of electrons between f orbitals are forbidden by the [|Laporte rule]. Furthermore, because of the "buried" nature of the f orbitals, coupling with molecular vibrations is weak. Consequently the spectra of lanthanide ions are rather weak and the absorption bands are similarly narrow. Glass containing [|holmium oxide] and holmium oxide solutions (usually in [|perchloric acid] ) have sharp optical absorption peaks in the spectral range 200–900 nm and can be used as a [|wavelength] calibration standard for [|optical spectrophotometers], [|[16]] and are available commercially. [|[17]] As f-f transitions are Laporte-forbidden, once an electron has been excited, decay to the ground state will be slow. This makes them suitable for use in [|lasers] as it makes the [|population inversion] easy to achieve. The [|Nd:YAG laser] is one that is widely used. Europium-doped yttrium vanadate was the first [|red phosphor] to enable the development of color television screens. [|[18]] Lanthanide ions have notable luminescent properties due to their unique 4f orbitals. Laporte forbidden f-f transitions can be activated by excitation of a bound "antenna" ligand. This leads to sharp emission bands throughout the visible, NIR, and IR and relatively long luminescence lifetimes. [|[19]]