Metallic radii. 178-9 properties

In bulk form cerium is a reactive metal that has a high affinity for oxygen and sulfur. It has a face centered cubic crystal stmcture, mp 798°C, bp 3443°C, density 6.77 g/mL, and a metallic radius of 182 pm. Detailed chemical and physical property information can be found in the Hterature (1,2). 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

Thus, although it is safe to assume that the 5f states are localized states, excited f states are never far above the Fermi energy. They will, therefore, influence the electronic properties and high temperature phases of Cf and Es (which, with a metal radius R = 2.0 A seem to have attained divalency). 

This treatment aiming to evaluate thermodynamically the orbital character of the bond in actinide metals, follows closely the general features illustrated above and has a particular value inasmuch as it is accompanied by a fairly comprehensive survey of the chemical and physical properties of actinide metals known at that time. In it, the metallic radius and the crystal structures are taken as valence indicators AH nd Tm as the bonding indicators . The metallic valence, however, is not taken as constant throughout the actinide series, but rather allowed to vary. The particular choices are justified by physical and chemical arguments, which are taken in support of the hypothesis chosen. 

Ionic radius Dy,+ 0.908 A. Metallic radius 1.775 A. First ionization potential 5.93 eV second 11.67 eV. Dysprosium appears to he exclusively trivalcnt. Other important physical properties of dysprosium arc given under Rare-Earth Elements and Metals. 

Electronic configuration 1. v22522/763 23/7 3 l04 24/764 /5.s 1. Ionic rad ius Ru4+ 0,60 A. Metallic radius 1,3251 A. First ionization potential 7.5 eV. Other physical properties of ruthenium will be found under Platinum and Platinum Group. See also Chemical Elements,... 

H2O + 2e, 1.216 V. Electron configuration l 22y22pa3s23pic,4r2. Ionic radius Zn+2 0.75 A. Metallic radius 1.33245 A. Other physical properties of zinc are described under Chemical Elements. 

Ionic radius Yb2+ 1.00 A Yb+J 0.88 A. Metallic radius 1.940A. First ionization potential 6.25 eV second 12.18 eV. Other important physical properties ufytterbium are given under Rare-Earlh Elements anti Metals. 

A correlation [93] relating crystal entropy to metallic radius, atomic wei t, magnetic properties, and electronic structure has permitted the accurate calculation of unknown entropies for these elements. This approach does require a defined electronic structure in order to predict accurate entropy values. Thermodynamics for the transplutonium metals have been sumnmrized [94,95]. 

PRACTICE EXAMPLE A Francium (Z = 87) is an extremely rare radioactive element formed when actinium (Z = 89) undergoes alpha-particle emission. Francium occurs in natural uranium minerals, but estimates are that little more than 15 g of francium exists in the top 1 km of Earth s crust. Few of francium s properties have been measured, but some can be inferred from its position in the periodic table. Estimate the melting point, density, and atomic (metallic) radius of francium. [Hint Plot each property versus atomic number, Z, and extrapolate to Z = 87.]... 

The data in Table 7.1 show that, as expected, density, ionic radius, and atomic radius increase with increasing atomic number. However, we should also note the marked differences in m.p. and liquid range of boron compared with the other Group III elements here we have the first indication of the very large difference in properties between boron and the other elements in the group. Boron is in fact a non-metal, whilst the remaining elements are metals with closely related properties. 

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