Lanthanide series oxidation states

Correspondingly, since in the second half of the actinide series (i.e., higher actinides) the energy required for the conversion of 4/ 5d should show more lower oxidation states such as + 2. Like lanthanides + 3 oxidation state is more common in actinides. 

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

Uranium is the fourth element of the actinide (SJ series. In the actinide series the electrons are more effectively shielded by the Is and 7p electrons relative to the 4f electrons (shielded by 6s, 6p) in the lanthanide (4p series. Thus, there is a greater spatial extension of 5f orbitals for actinides than 4f orbitals for lanthanides. This results in a small energy difference between and 5/ 6d7s electronic configurations, and a wider range of oxidation states is... 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

This state is found for the six elements Am and Cf No, though in aqueous solution only for Fm, Md and No. However, for No, alone amongst all the f-series elements, it is the normal oxidation state in aqueous solution. The greater stabilization of the +2 state at the end of the actinides as compared to that at the end of the lanthanides which this implies, has been taken " to indicate a greater separation between the 5f and 6d than between the 4f and 5d orbitals at the ends of the two series. This is the reverse of the situation found at the beginnings of the series (p. 1266). 

The principle just outhned has two parts. The first part deals with redox processes and was developed here by examining the relative stabihties of the -i-2 and -i-3 oxidation states of the lanthanides. It can be extended in a variety of ways. Thus if the f variation is shifted one element to the right, it tells us the nature of the f variations, and accounts for the distribution of the -i-4 oxidation states of the lanthanides [2, 10, 15]. Their stability shows maxima at cerium(IV) and terbium(IV), decreasing rapidly as one moves from these elements across the series. 

Lanthanide fluoride sequences show monotonic increase in bond energy with increasing oxidation state, although one would expect a maximum for EuF2 in the europium series at the stable f7 configuration. 

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

The ores from which rare-earth elements are extracted are monazite, bastnasite, and oxides of yttrium and related fluorocarbonate minerals. These ores are found in South Africa, Australia, South America, India, and in the United States in Cahfomia, Florida, and the Carolinas. Several of the rare-earth elements are also produced as fission by-products during the decay of the radioactive elements uranium and plutonium. The elements of the lanthanide series that have an even atomic number are much more abundant than are those of the series that have an odd atomic number. 

SYMBOL Eu PERIOD 6 SERIES NAME Lanthanide ATOMIC NO 63 ATOMIC MASS 151.964 amu VALENCE 2 and 3 OXIDATION STATE +2 and +3 NATURAL STATE Solid ORIGIN OF NAME Named for the continent of Europe. 

Symbol Gd atomic number 64 atomic weight 157.25 a lanthanide series rare earth element electron configuration 4/ 5di6s2 partially filled / orbital common oxidation state -i-3 six stable natural isotopes Gd-152 (0.2%), Gd-154 (2.86%), Gd-155 (15.61%, Gd-156 (20.59%), Gd-157 (16.42%), Gd-157 (23.45%)... 

Symbol La atomic number 57 atomic weight 138.91 a rare-earth transition metal, precursor to a series of 14 inner-transition elements known as the lanthanide series electron configuration [XejSdiGs oxidation state -i-3 atomic radius 1.879A ionic radius (LaS+) 1.061A electronegativity 1.17 two natural isotopes are La-139 (99.911%) and La-138 (0.089%). 

On the contrary, there is a spread of oxidation numbers for the light actinides (at least up to Cm), which, for Pu and Np, range from 3 to 7 After Cm, however, the trivalent oxidation state is always met, and this second half of the actinide series approaches more the behaviour of the lanthanides. 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

In spite of considerable similarities between the chemical properties of lanthanides and actinides, the trivalent oxidation state is not stable for the early members of the actinide series. Due to larger ionic radii and the presence of shielding electrons, the 5f electrons of actinides are subjected to a weaker attraction from the nuclear charge than the corresponding 4f electrons of lanthanides. The greater stability of tetrapositive ions of actinides such as Th and Pu is attributed to the smaller values of fourth ionization potential for 5f electrons compared to 4f electrons of lanthanides, an effect that has been observed in aqueous solution of Th and Ce (2). Thus, thorium... 

Evidently the most - practically the only - stable oxidation state of La in ionic compounds is III. Does this hold for the later members of the lanthanide series Fig. 5.1 suggests that the I oxidation state has little prospect of stability, given the high atomisation enthalpies and the relatively low second and third ionisation energies. The II oxidation state has better prospects, however. Consider the disproportionation ... 

We now turn to the 3d series elements. The dihalides and trihalides can be treated as ionic solids, although the chlorides, bromides and iodides adopt layer structures which might be better viewed as polymeric covalent crystals. In Fig. 5.2 the third ionisation energies of the 3d atoms are plotted alongside those of the lanthanides. These all involve the removal of an electron from a 3d orbital from Fe onwards, the orbital concerned is doubly occupied so that spin-pairing energy assists the ionisation. This accounts for the break between Mn and Fe, as previously discussed (Section 4.3). The increase from Sc to Mn, and from Fe to Zn, is much sharper than the corresponding increases in the lanthanide series. However, the break at the half-filled shell is less abrupt for the 3d series. This explains why the II oxidation state - which is... 

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