Lanthanum atomic radius

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. 

Use the data from Appendix E to graph the variation of atomic radius with atomic number for the rare-earth elements from lanthanum to lutetium. 

Abstract This chapter discusses the chemical and physical properties of the lanthanides, some of which are in a certain way peculiar. It discusses the oxidation states of the REE, and the phenomenon called the lanthanide contraction (meaning that the atomic radius decreases with increasing atomic number in the series lanthanum-lutetium). It lists the isotopes known per element, and explains the radioactivity of promethium, the only element of the rare earths that has only radioactive isotopes and no stable isotopes. Magnetism and luminescence also are discussed. 

In the periodic system, the lanthanide group of elements also gives rise to a peculiar phenomenon, called the lanthanide contraction. This phenomenon is the important and progressive decrease in atomic radii and in radii of ions when going from lower to higher atomic numbers in the lanthanide series. Thus lanthanum has the largest atomic radius, and lutetium has the smallest. In Table 3.3, the ionic radii for the lanthanides are given, and the effect described above can be clearly seen in Fig. 3.2. 

Assume that the van der Waals radius of the lanthanum atom is about 3.5 A. Use the carbon-carbon bond lengths in Table 5.1 to estimate the minimum number of interconnected, sp hybridized, and aromatic carbon atoms that would be required to completely encase one lanthanum atom in a roughly spherical shell. 

In the sixth period, the/-block elements fall between lanthanum (Group 3) and hafnium (Group 4). Because of the increase in atomic number that occurs firom lanthanum to hafnium, the atomic radius of hafnium is actually sUghtiy less than that of zirconium, Zr, the element immediately above it The radii of elements following hafnimn in the sixth period vary with increasing atomic number in the usual maimer. 

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

Comparison of the predictions of this scheme with the data for the diamagnetic sesquioxides of lanthanum and lutetium (193) suggests that if all of the entropy variation is to be ascribed to the cation, the contributions would have to decrease with increasing mass (or atomic number) from 15.2 for lanthanum to 13.0 for lutetium. As an approximation, the decrease is taken proportional to that of the cationic radius obtained by x-ray diffraction measurements. The following (improved) values then apply ... 

The lanthanide or rare earth elements (atomic numbers 57 through 71) typically add electrons to the 4f orbitals as the atomic number increases, but lanthanum (4f°) is usually considered a lanthanide. Scandium and yttrium are also chemically similar to lanthanides. Lanthanide chemistry is typically that of + 3 cations, and as the atomic number increases, there is a decrease in radius for each lanthanide, known as the lanthanide contraction. Because bonding within the lanthanide series is usually predominantly ionic, the lanthanide contraction often determines the differences in properties of lanthanide compounds and ions. Lanthanide compounds often have high coordination numbers between 6 and 12. see also Cerium Dysprosium Erbium Europium Gadolinium Holmium Lanthanum Lutetium Praseodymium Promethium Samarium Terbium Thulium Ytterbium. 

Fractional crystallization works best for the lanthanum end of the series, as there the differences in ionic radius are the largest. Fractional crystallization is very slow for the heavy rare earths and in the Sm(ni)-Gd(III)—region, because the differences in properties between the rare earths decrease as the atomic number increases (Gupta and Krishnamurthy 2005). 

Fignre 23.3 compares the covalent radii of the transition elements. The atomic radii increase in going from a fourth-period to a fifth-period element within any column. For example, reading down the Group IIIB elements, you find that the covalent radius of scandium is 144 pm and of yttrium is 162 pm. You would expect an increase of radius from scandium to yttrium because of the addition of a shell of electrons. Continuing down the column of lllB elements, you find a small increase of covalent radius to 169 pm in lanthanum. But all of the remaining elements of the sixth period have nearly the same covalent radii as the corresponding elements in the fifth period. 

The Lai-Lai distances within the planar metal sheets are extremely short compared with the distances in metallic lanthanum (3.73 and 3.77 A). In the Cu2Sb-type compounds the radius ratios rm/rx are generally —0.8 0.9 so that the atoms in the planar layer are not close packed. In La2Sb, however, r lrx = 1.15 so that the Lai atoms have to be compressed by —12% (Stassen et al., 1970). 

The concepts derived from atomic spectra have been very important in the recent progress of understanding spectroscopic properties and such chemical questions as the deviations of the oxidation state from M(III) and the conditions for metallic character of the compounds. We return to these individual properties specifically dependent on 4f in section 2, and we start with the smoothly varying properties which can be described as if the lanthanide M(III) is a sphere of electronic density gradually decreasing its radius from lanthanum to lutetium. The contributions of quantum chemistry to this, apparently simpler problem, have been much more qualitative than the specifically spectroscopic statements. 

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