Actinoids 336

Actinoids or actinides (Ac to Lr, those whose 5/ shell is being filled)... 

Hydrogen combines with many elements to form binary hydrides MH (or M H ). All the main-group elements except the noble gases and perhaps indium and thallium form hydrides, as do all the lanthanoids and actinoids that have been studied. Hydrides are also formed by the more electropositive transition elements, notably Sc, Y, La, Ac Ti, Zr, Hf and to a lesser... 

The carbides of the lanthanoids and actinoids can be prepared by heating M2O3 with C in an electric furnace or by arc-melting compressed pellets of the elements in an inert atmosphere. They contain the C2 unit and have a stoichiometry MC2 or M4(C2)3. MC2 have the CaC2 structure or a related one of lower symmetry in which the C2 units lie at right-angles to the c-axis of an orthogonal NaCl-type cell. They are more reactive than the alkaline-earth metal... 

The lanthanoids also form metal-rich carbides of stoichiometry M3C in which individual C atoms occupy at random one-third of the octahedral Cl sites in a NaCl-like structure. Several of the actinoids (e.g. Th, U, Pu) form monocarbides, MC, in which all the octahedral Cl sites in the NaCl structure are occupied and this stoichiometry is also observed for several other carbides of the early transition elements, e.g. M = Ti, Zr, Hf V, Nb, Ta Mo, W. These... 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

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. 

In some versions of the periodic table, the lanthanoids begin with cerium (element 58) and the actinoids begin with thorium (element 90). 

FIGURE 1.38 The seven /-orbitals of a shell (with n = 3) have a very complex appearance. Their detailed form will not be used again in this text. However, their existence is important for understanding the periodic table, the presence of the lanthanoids and actinoids, and the properties of the later d-block elements. A darker color denotes a positive lobe, a lighter color a negative lobe. 

Difficulties in separating and isolating the lanthanoids delayed their widespread use in technology. However, today they are studied intensely, because superconducting materials often contain lanthanoids (Fig. 1.64). All the actinoids are radioactive. None of the elements following plutonium occurs naturally on Earth in any significant amount. Because they can be made only in nuclear reactors or particle accelerators, they are available only in small quantities. 

In the heavier transition-metal elements, especially the lanthanoids and actinoids, there are numerous exceptions to the regular order of orbital occupation predicted by the building-up principle. Suggest why more exceptions would be noted for these elements. 

Group IIIA (3). Scandium, Yttrium, Lanthanoids, Actinoids... 

The actinoid elements (or actinides An) constitute a series of 14 elements which are formed by the progressive filling of the 5/ electron shell and follow actinium in the periodic table (atomic numbers 90-103). All of the isotopes of the actinide elements are radioactive and only four of the primordial isotopes, Th, and " " Pu, have a sufficient long half-life for there to be any of these left in nature. 

Numerous compounds adopt the PbFCl structure. These include, apart from fluoride chlorides, oxide halides MOX (M = Bi, lanthanoids, actinoids X = Cl, Br, I), hydride halides like CaHCl and many compounds with metallic properties like ZrSiS or NbSiAs. 

A The Periodic Table can also be ordered according to the electrons in the outer orbital. This makes the "inner occupation" in the case of the transition metals and the lanthanoids and actinoids particularly clear. 

Name named after Pluto, which recently lost its status as a planet Greek god of the underworld and wealth G. T. Seaborg recognized the analogy between the lanthanoids and actinoids... 

The rules above gave maximum and minimum oxidation numbers, but those might not be the only oxidation numbers or even the most important oxidation numbers for an element. Elements of the last six groups of the periodic table for example may have several oxidation numbers in their compounds, most of which vary from each other in steps of 2. For example, the major oxidation states of chlorine in its compounds are -1, +1, +3, +5, and +7. The transition metals have oxidation numbers that may vary from each other in steps of 1. The inner transition elements mostly form oxidation states of + 3, but the first part of the actinoid series acts more like transition elements and the elements have... 

Since 1975, porphyrin complexes of all metals of the periodic table of elements are known, with the exception of some actinoids (20-22). The most fascinating property of all metalloporphyrins is their intense color porphyrlike in the solid state, brick-red, pinkish red, olive-green, or brown in dilute solutions. The absorption spectra vary with the metal, the porphyrin, and the axial ligand many attempts to rationalize the different types of spectra have been undertaken (20-22, 24, 42-48). As the. .periodic table of metalloporphyrins (24) is now nearly complete, a new approach... 

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