Lanthanides . The lanthanide series is the group of chemical elements that follow lanthanum in group IIIB of the periodic table . Their atomic distinction is that they occupy 4f electronic sublevel. Initially, only these elements with atomic numbers 58 to 71 are lanthanides.
Summary
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- 1 Physico-chemical characteristics
- 2 Natural state and separation
- 3 See also
- 4 External links
- 5 Sources
Physicochemical characteristics
The physical and chemical properties of lanthanides are remarkably similar to the point that they cannot be separated by ordinary chemical methods of selective precipitation but by laborious fractional crystallization.
The elements can be grouped into three families:
- that of Cerium[ Cerium (Ce 58), Praseodymium (Pr 59), Neodymium (Nd 60), Prometium (Pm 61) and Samarium (Sm 62)], which form insoluble double sulfates
- that of Terbium[ Europium (Eu 63), Gadolinium (Gd 64) and Terbium (Tb 65)], which give moderately soluble double sulfates
- that of Yttrium[ Dysprosium (Dy 66), Holmium (67 Ho), Erbium (Er 68), Tulium (Tm 69), Itherbium (Yb 70) and Lutetium (Lu 71)], whose double sulfates are highly soluble
Natural state and separation
Lanthanide elements were originally known as rare earths due to their presence in oxide mixtures . They are not rare elements and their absolute abundance in the lithosphere is relatively high.
Even the rarest, Thulium , is as common as Bismuth and more ordinary than Ace, Se, Hg, and Cd, which are generally not considered rare. Large deposits exist in Scandinavia , India , Russia and the USA. ; the many smaller tanks are distributed in many other places. These deposits are made up of many minerals, with monazite being one of the most important and it is almost always presented as a heavy and dark sand of variable composition. Monazite is essentially a lanthanide orthophosphate, but significant amounts of Th, above 30%, are found in most monazite sands.
The individual distribution of lanthanides in minerals is such that the general La, Ce, Pr and Nd constitute approximately 90%, the rest being formed by Y together with the heaviest elements. The most abundant is cerium 0.00031%, followed by neodymium 0.00018%. Monazite and other lanthanide-containing minerals in the +3 oxidation state are generally poor in Eu, which due to its relatively strong tendency to give the +2 state, is most often concentrated in Group II minerals.
In nature, only traces of Prometium are found in Uranium minerals as a fragment resulting from spontaneous fission 238 U. By means of ion exchange methods, milligram amounts of salts of 147 Pm + can be separated from the products of fission of the reactors where the 147 Pm is formed (b – , 2.64 years).
Lanthanides are separated from most other elements by precipitation of oxalates or fluorides from nitric acid solutions . The elements are separated from each other by ion exchange, which for commercial purposes is carried out on a large scale. In general, Ce and Eu are separated initially, the first by oxidation to Ce +4 and then precipitating IO 3 – which is insoluble in HNO 3 6N or by solvent extraction; the Eu is separated by reduction to Eu +2, then precipitating it as EuSO 4 .
The behavior in the ion exchange process depends mainly on the hydrated ionic radius, and the lanthanum must be bound more closely than the Lu; therefore, the elution order is Lu ® La.
Despite the fact that Ce +4 (also Th +4 and Pu +4 ) is easily extracted from HNO 3 solutions with tributyl phosphate in kerosene, trivalent lanthanide nitrates can also be extracted under appropriate conditions, using various acid phosphate esters. The ease of extraction under certain conditions increases with increasing Z; it is greater in strong acids or in high nitrate concentrations.
