Lanthanides chemical properties

Thus it can be seen that elements in and near the island of stabiHty based on element 114 can be predicted to have chemical properties as foUows. Element 114 should be a homologue of lead, that is, should be eka-lead, and element 112 should be eka-mercury, element 110 should be eka-platinum, etc (26,27). If there is an island of stabiHty at element 126, this element and its neighbors should have chemical properties like those of the actinide and lanthanide elements (26). 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

The reason usually cited for the great similarity in the properties of the lanthanides is that they have similar electronic configurations in the outermost 6s and 5d orbitals. This occurs because, at this point in the periodic table, the added electrons begin to enter 4f orbitals which are fairly deep inside the atom. These orbitals are screened quite well from the outside by outer electrons, so changing the number of 4/electrons has almost no effect on the chemical properties of the atom. The added electrons do not become valence electrons in a chemical sense—neither are they readily shared nor are they readily removed. 

A predominant feature of the atomic structure of the lanthanide group is the sequential addition of 14 electrons to the 4f subshell (Table 1). The /"electrons do not participate in bond formation and in ordinary aqueous solutions all of the lanthanides exhibit a principal (III) state. The common (III) state confers a similarity in chemical properties to all lanthanide elements. Some of the lanthanides can also exist in the (II) state (Nd, Sm, Eu, Tm, Yh) or in the (IV) state (Ce, Pr, Nd, Tb, Dy). Except for Ce(IV), Eu(II), and Yb(II), these unusual lanthanide oxidation states can only be prepared under drastic redox pressure and temperature conditions, and they are not stable in aqueous solutions. Cerium (IV) is a strong oxidizing agent... 

The higher coordinating ability and Lewis acidity of Zn(H) ion in addition to the low pK of the metal-bound water molecule and the appearance of this metal ion in native phosphatases inspired a number of research groups to develop Zn(II)-containing dinuclear artificial phosphatases. In contrast, very few model compounds have been published to mimic the activity of Fe(III) ion in dinuclear centers of phosphatase enzymes. Cu(II) or lanthanide ions are not relevant to natural systems but their chemical properties in certain cases allow extraordinarily high acceleration of phosphate-ester hydrolysis [as much as 108 for copper(II) or 1013 for lanthanide(III) ions]. 

Lanthanides properties and general references. For a systematic treatment and general references of the physical and chemical properties of the rare earths and their compounds and alloys mention can be made to a periodical publication in which several contributions to these subjects are being collected. See for instance Gschneidner and Eyring (1978) and Gschneidner etal. (2005). We would also like to quote a sentence, included in the prefaces of all these books, which hints at the complexity and richness of the rare earth behaviour and the ever-increasing interest in their properties and applications. The mentioned sentence is as follows ... 

Figure 5.9. Lanthanide and actinide chemical properties. A scheme is shown of the oxidation states they present in their various classes of compounds. A rough indication of a greater frequency and a higher relative stability of each state is given by the darker blackening of each box. Notice the overwhelming presence of oxidation state 3, in the lanthanides and heavy actinides, oxidation state 2 in Eu andYb and of several higher oxidation states in U and nearby elements.

The atomic spectra of the actinides are very complex and it is difficult to identify levels in terms of quantum numbers and configurations (6). The chemical behaviour of the elements is dictated by the configurations of the electrons around the nucleus and in the case of the actinides it is the competition between the 5/ 1 7 s2 and the 5 /n 1 6 d 7 s2 levels that dictates these chemical properties. A comparison of the /-energy levels of the lanthanides and the actinides shows that less energy is required for the promotion of the 5 / -> 6 d levels than for the 4/ -> 5 d levels in the lanthanides. As a result of this lower energy requirement by the actinides they have the tendency to display higher valences since the bonding electrons are more readily available. It is only at the commencement of the second half of the actinides that there is commencement of properties which echo those of the lanthanides. 

The lanthanide group of elements (Table 11.7) is very difficult to separate by traditional methods because of their similar chemical properties. The techniques originally used, like the precious metals, included laborious multiple fractional recrystallizations and fractional precipitation, both of which required many recycle streams to achieve reasonably pure products. Such techniques were unable to cope with the demands for significant quantities of certain pure compounds required by the electronics industry hence, other separation methods were developed. Resin ion exchange was the first of these... 

The elements in the lanthanide series are also called rare-earth elements they are not scarce or rare, but at one time they were thought to be rare because they were very difficult to find and extract from their ores, difficult to separate from each other, and difficult to identify. Chemical elements that have similar physical and chemical properties tend to occur together in the same ores and minerals. 

The lanthanides have, as known, very similar chemical properties across the series. Writing them in a single separate line in the periodic chart intends to convey this information to the reader. 

Experimental investigations of spectroscopic and other physical-chemical properties of actinides are severely hampered by their radioactive decay and radiation which lead to chemical modifications of the systems under study. The diversity of properties of lanthanide and actinide compounds is unique due to the multitude of their valency forms (which can vary over a wide range) and because of the particular importance of relativistic effects. They are, therefore, of great interest, both for fundamental research and for the development of new technologies and materials. The most important practical problems involve storage and processing of radioactive waste and nuclear fuel, as well as pollution of the environment by radioactive waste, where most of the decayed elements are actinides. 

The first genuine transuranic element was discovered at Berkeley, where Edwin McMillan used Lawrence s cyclotron in 1939 to bombard uranium with slow neutrons. He saw beta decay from what he predicted was element 93, and set about trying to isolate it. McMillan saw that the element sits beneath the transition metal rhenium in the Periodic Table, and so he assumed it should share some of rhenium s chemical properties. But when he and Fermi s one-time collaborator Emilio Segre performed a chemical analysis, they found that eka-rhenium (in Mendeleyev s terminology) behaved instead like a lanthanide, the series of fourteen elements that loops out of the table after lanthanum (see page 152). Disappointed, they figured that all they had found was one of these known elements. 

The periodicity of chemical properties arises from filling of successive quantum mechanical shells of electrons. For example, filling of the s,p shells, with capacities of 8 electrons each, and the d shells, which can hold up to 10 electrons, is associated with the main group and transition elements, respectively (Fig. 1.1). Before the advent of quantum theory, two classes of elements were known that seemed not to fit the Mendeleyevian scheme an uncertain number of rare earth elements or lanthanides— metallic elements, discovered throughout the 1800s, that form oxides of... 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

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