Lanthanide electron shells

A contraction resulting from the filling of the 4f electron shell is of course not exceptional. Similar contractions occur in each row of the periodic table and, in the d block for instance, the ionic radii decrease by 20.5 pm from Sc to Cu , and by 15 pm from Y to Ag . The importance of the lanthanide contraction arises from its consequences ... 

Figure 1.1 Principal features of the periodic table. The International Union of Pure and Applied Chemistry (IUPAC) now recommends Arabic group numbers 1-18 in place of the traditional Roman I—VIII (A and B). Group names include alkali metals (1), alkaline earth metals (2), coinage metals (11), chalcogens (16), and halogens (17). The main groups are often called the s,p block, the transition metals the d, block elements, and the lanthanides and actinides the / block elements, reflecting the electronic shell being filled. (See inside front cover for detailed structure of the periodic table.)...

Like the transition metals, however, the lanthanides and actinides break the rules a little when it comes to their valence electron shell. Transition metals share electrons from the d orbital in their next-to-outermost shell. The valence electrons in lanthanides... 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

The effect of filling the lanthanide f-shell on gas ion size is simply due to internal 4/ electron-electron repulsion which is reflected in the L/S coupling, i.e. the magnetic... 

Filling of the inner 4f electron shell across the lanthanide series results in decreases of ionic radii by as much as 15% from lanthanum to lutetimn, referred to as the lanthanide contraction (28). While atomic radius contraction is not rmique across a series (i.e., the actinides and the first two rows of the d-block), the fact that all lanthanides primarily adopt the tripositive oxidation state means that this particular row of elements exhibits a traceable change in properties in a way that is not observed elsewhere in the periodic table. Lanthanides behave similarly in reactions as long as the mnnber of 4f electrons is conserved (29). Thus, lanthanide substitution can be used as a tool to tune the ionic radius in a lanthanide complex to better elucidate physical properties. 

The elements with atomic numbers from 57 (l thanum) to 71 (lutetium) are referred to as the lanthanide elements. These elements and two others, scandium and yttrium, exhibit chemical and physical properties very similar to lanthanum. They are known as the rare earth elements or rare earths (RE). Such similarity of the RE elements is due to the configuration of their outer electron shells. It is well known that the chemical and physical properties of an element depend primarily on the structure of its outermost electron shells. For RE elements with increasing atomic number, the first electron orbit beyond the closed [Xe] shell (65 remains essentially in place while electrons are added to the inner 4f orbital. Such disposition of electrons about the nucleus of the rare earth atoms is responsible for the small effect an atomic number increase from 57 to 71 has on the physical and chemical properties of the rare earths. Their assignment to the 4f orbital leads to slow contraction of rare earth size with increasing atomic number. The 4f orbitals of both europium and gadolinium are half occupied [Xe] (4F6s and [Xe] (4F5d 6s, so that there... 

Oxides of the lanthanide rare earth elements share some of the properties of transition-metal oxides, at least for cations that can have two stable valence states. (None of the lanthanide rare earth cations have more than two ionic valence states.) Oxides of those elements that can only have a single ionic valence are subject to the limitations imposed on similar non-transition-metal oxides. One actinide rare-earth oxide, UO2, has understandably received quite a bit of attention from surface scientists [1]. Since U can exist in four non-zero valence states, UO2 behaves more like the transition-metal oxides. The electronic properties of rare-earth oxides differ from those of transition-metal oxides, however, because of the presence of partially filled f-electron shells, where the f-electrons are spatially more highly localized than are d-electrons. 

Also shown in this version of the Periodic Table is the atomic number of each element, which corresponds to the total number of electrons, and the atomic weight relative to the mass of which has been assigned a mass of 12.000 (the atomic weight of carbon shown in the Periodic Table is slightly higher than this because of the additional presence of a small amount of the isotope in natural carbon). The atomic weight represents the sum of the numbers of protons and neutrons in the nucleus of the atom. It has long been known that the elements in a vertical column have similar chemical properties because they have the same nornber of valence electrons. However, the lanthanides and actinides (except for thorium) Ihown at the bottom of the Table do not fit readily into this scheme because of the effect of/orbitals in the outer electron shells. 

The absence of 5d electrons and the inertness of the lanthanides 4f shell makes n backbonding energetically unfavourable and simple carbonyls, for instance, have only been obtained in argon matrices at 8-12K. On the other hand, essentially ionic cyclopentadienides are well known and an increasing number of u-bonded Ln-C compounds have been produced (see section 30.3.5). 

In other experiments, M ly observed the complete reduction of Md + with but the reduction was incomplete when Ti + was used (21). From these observations, he concluded the standard reduction potential of Md + was close to -0.1 volt. The standard potentials obtained by both groups are in reasonable agreement and, most importantly, they conclusively show that the stability of Md + is greater than any lanthanide(II) ion. This finding was surprising since divalency in the lanthanides is mainly associated with the special stability given by the half-filled and fully-filled -electron shell. Divalent Md ions are at least one electron short of the stable 5f- configuration. 

The actinide element series, like the lanthanide series, is characterized by the filling of an f-electron shell. The chemical and physical properties, however, are quite different between these two series of f-electron elements, especially in the first half of the series. The differences are mainly due to the different radial extension of the 4f- and 5f-electron wavefunctions. For the rare-earth ions, even in metallic systems, the 4f electrons are spatially well localized near the ion sites. Photoemission spectra of the f electrons in lanthanide elements and compounds always show "final state multiplet" structure (3), spectra that result from partially filled shells of localized electrons. In contrast, the 5f electrons are not so well localized. They experience a smaller coulomb correlation interaction than the 4f electrons in the rare earths and stronger hybridization with the 6d- and 7s-derived conduction bands. The 5f s thus... 

The seventh period of the periodic table is occupied by a similar series called the actinide series. Beginning with actinium the 5/ electron shell is populated in a matmer analogous to filling the 4/ electron shell in the lanthanide series. A suggested electronic configuration [K2, M6], is shown in Table 9.2. After the alkaline earth radium, additional electrons are added to the 6d and 5/ shells, beginning the actinide series. At the beginning of the actinide series electrons are added... 

There are also various combinations of the above, and as the atomic weight of the atom increases, the above selection rules do not hold rigorously, because the radiation states become "mixed", i.e.- they are not pure "spin-orbit" coupUng states any more. That is, L-S coupling does not hold. For this reason, prediction of spectral intensities by selection rules is not exact for heavy atoms, as we shall see for rare earth (lanthanide) cations. Note that the above description applies only to inorganic cations, where the electronic transitions (excitation) occur within the outer electron shells. Phosphors based on organic compounds show quite different mechanisms. 

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