Lanthanum Element

Naturally occurring lanthanum (La) has one stable ( La) and one radioactive ( La) isotope. The stable isotope La is the most abundant (99.91 % natural abundance). In total, 38 radioisotopes have been found. The most stable ones are La with a half-life of 102 x 10 years, La with a half-life of 60,000 years and with a half-life of 1.6781 days. The remaining radioactive isotopes have half-lives that are less than a day. The majority of the half-lives of these isotopes are less than 1 min (Table 3.4). 

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Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

The members of the d block, with the exception of the elements in Group 12 (the zinc group) are called transition metals. As we shall see, these elements are transitional in character between the vigorously reactive metals in the s block and the less reactive metals on the left of the p block. The members of the f block, which is shown below the main table (to save space), are the inner transition metals. The upper row of this block, following lanthanum (element 57) in Period 6, consists of the lanthanides and the lower row, following actinium (element 89) in Period 7, consists of the actinides. 

The rare earth elements are called the lanthanoids, a name that comes from lanthanum (element 57). 

The element lanthanum (atomic number 57) has the electronic configuration... 

Reference has been made already to the existence of a set of inner transition elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the rare earths , since they occurred in uncommon mixtures of what were believed to be earths or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the lanthanons or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state -i-3 and show in this state predominantly ionic characteristics—the ions. 

Lanthanum was isolated in relatively pure form in 1923. Iron exchange and solvent extraction techniques have led to much easier isolation of the so-called "rare-earth" elements. 

Europium is now prepared by mixing EU2O3 with a 10%-excess of lanthanum metal and heating the mixture in a tantalum crucible under high vacuum. The element is collected as a silvery-white metallic deposit on the walls of the crucible. 

Gr. neos, new, and didymos, twin) In 1841, Mosander, extracted from cerite a new rose-colored oxide, which he believed contained a new element. He named the element didymium, as it was an inseparable twin brother of lanthanum. In 1885 von Welsbach separated didymium into two new elemental components, neodymia and praseodymia, by repeated fractionation of ammonium didymium nitrate. While the free metal is in misch metal, long known and used as a pyrophoric alloy for light flints, the element was not isolated in relatively pure form until 1925. Neodymium is present in misch metal to the extent of about 18%. It is present in the minerals monazite and bastnasite, which are principal sources of rare-earth metals. 

Lanthanides is the name given collectively to the fifteen elements, also called the elements, ranging from lanthanum. La, atomic number 57, to lutetium, Lu, atomic number 71. The rare earths comprise lanthanides, yttrium, Y, atomic number 39, and scandium. Sc, atomic number 21. The most abundant member of the rare earths is cerium, Ce, atomic number 58 (see Ceriumand cerium compounds). 

High Temperature Corrosion. The rate of oxidation of magnesium adoys increases with time and temperature. Additions of berydium, cerium [7440-45-17, lanthanum [7439-91-0] or yttrium as adoying elements reduce the oxidation rate at elevated temperatures. Sulfur dioxide, ammonium fluoroborate [13826-83-0] as wed as sulfur hexafluoride inhibit oxidation at elevated temperatures. 

Some elements found in body tissues have no apparent physiological role, but have not been shown to be toxic. Examples are mbidium, strontium, titanium, niobium, germanium, and lanthanum. Other elements are toxic when found in greater than trace amounts, and sometimes in trace amounts. These latter elements include arsenic, mercury, lead, cadmium, silver, zirconium, beryUium, and thallium. Numerous other elements are used in medicine in nonnutrient roles. These include lithium, bismuth, antimony, bromine, platinum, and gold (Eig. 1). The interactions of mineral nutrients with... 

Some nut trees accumulate mineral elements. Hickory nut is notable as an accumulator of aluminum compounds (30) the ash of its leaves contains up to 37.5% of AI2O2, compared with only 0.032% of aluminum oxide in the ash of the Fnglish walnut s autumn leaves. As an accumulator of rare-earth elements, hickory greatly exceeds all other plants their leaves show up to 2296 ppm of rare earths (scandium, yttrium, lanthanum, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). The amounts of rare-earth elements found in parts of the hickory nut are kernels, at 5 ppm shells, at 7 ppm and shucks, at 17 ppm. The kernel of the Bra2d nut contains large amounts of barium in an insoluble form when the nut is eaten, barium dissolves in the hydrochloric acid of the stomach. 

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

Compared to later elements in their respective transition series, scandium, yttrium and lanthanum have rather poorly developed coordination chemistries and form weaker coordinate bonds, lanthanum generally being even less inclined to form strong coordinate bonds than scandium. This is reflected in the stability constants of a number of relevant 1 1 metal-edta complexes ... 

To avoid this confusion, and because many of the elements are actually far from rare, the terms lanthanide , lanthanon and lanthanoid have been introduced. Even now, however, there is no general agreement about the position of La, i.e, whether the group is made up of the elements La to Lu or Ce to Lu. Throughout this chapter the term lanthanide and the general symbol, Ln, will be used to refer to the fourteen elements cerium to lutetium inclusive, the Group 3 elements, scandium, yttrium and lanthanum having already been dealt with in Chapter 20. 

The actinides ( actinons or actinoids ) are the fourteen elements from thorium to lawren-cium inclusive, which follow actinium in the periodic table. They are analogous to the lanthanides and result from the filling of the 5f orbitals, as the lanthanides result from the filling of the 4f. The position of actinium, like that of lanthanum, is somewhat equivocal and, although not itself an actinide, it is often included with them for comparative purposes. 

Other detrimental factors which should to be taken into account in the materials selection process include temperature cycling and the presence of halide gases. Specialist alloys containing rare earth element additions such as cerium, lanthanum and yttrium have been developed for use in certain environments up to 130°C. 

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