Metal metallic radius

Element Ionisation energy (kj mof ) Metallic radius (nm) Ionic radius (nm) Heal oj laporibation at 298 K (kJ mol ) Hydration energy oj gaseous ion (kJ moI ) (V)... 

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

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). 

The viscosities of liquid metals vaty by a factor of about 10 between the empty metals, and the full metals, and typical values are 0.54 x 10 poise for liquid potassium, and 4.1 x 10 poise for liquid copper, at dreir respective melting points. Empty metals are those in which the ionic radius is small compared to the metallic radius, and full metals are those in which the ionic radius is approximately the same as tire metallic radius. The process was described by Andrade as an activated process following an AiThenius expression... 

Figure 30.2 Variation of metal radius and 3+ ionic radius for La and the lanthanides. Other data for Ln" and Ln" are in Table 30.2.

It is interesting that a straight line drawn through the tetrahedral radii passes through the metallic radius for calcium this suggests that the metallic bonding orbitals for calcium are sp orbitals, and that those for scandium begin to involve d-orbital hybridization. 

The metallic radius for indium differs from that of gallium by being greater than the tetrahedral radius it lies nicely on a straight line... 

An equation has been formulated to express the change in covalent radius (metallic radius) of an atom with change in bond number (or in coordination number, if the valence remains constant), the stabilizing (bond-shortening) effect of the resonance of shared-electron-pair bonds among alternative positions being also taken into consideration. This equation has been applied to the empirical interatomic-distance data for the elementary metals to obtain a nearly complete set of single-bond radii. These radii have been compared with the normal covalent... 

Fig. 3. Curves showing values of the reciprocal of the metallic radius for ligancy 12 of metals of the sequences Ca to Ge, Sr to Sn, and Ba to Pb. The vertical scale has been shifted down for the second and third sequences by 0.1 and 0.2, respectively.

Dullenkopf, 1936 Riederer, 1936 Fink Willey, 1937 Little, Raynor Hume-Rothery, 1943). The approximate composition Mg3Zn3Al2 was assigned to the phase, which extends over a wide range of values of the Zn/Al ratio. The atomic percentage of magnesium is nearly constant for the alloys, as would be expected from the fact that the metallic radius of magnesium is about 15% greater than those of aluminum and zinc. 

Approximate atomic coordinates were obtained by assuming the effective metallic radius of magnesium to be about 1-60 A and the radii of aluminum and zinc to be about 1-40 A. The corresponding calculated structure factors were in fairly good agreement with those obtained from the observed intensities. The preliminary atomic coordinates are given in Table 1. 

Fig. 1.—Values of the metallic radius for ligancy 12, represented by circles, for the elements yttrium to silver. The straight line represents the expected values for valence of other metals equal to the valence of molybdenum that is, it shows the effect of increasing nuclear charge only.

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. 

The distribution of the observed higher borides among the five structural types (MB2, MB4, MBg, MB]2 and Mg ) presented in Table 1, which shows correlations with the metallic radius r. values of which are in order of decreasing magnitude (r, corresponds to coordination number 12). In order to discuss the existence of the actinide borides, the table also shows the unit cell volume V of the borides MB4, MBg and MB,2. 

Among the tetraborides, UB4 has the smallest volume and hence the smallest effective radius. Thus an actinide element having a metallic radius of 1.59 A (Pu) or smaller forms a diboride, while those having larger radii do not. As in the rare-earth series, the actinides able to form MB4, MBg and MB,2 borides form also MB2 diborides (Table 1). 

Although Zr and Sc have close metallic radii, the former has the UB,2-type structure and the latter exhibits a tetragonal symmetry, but its structure is not Imown. This indicates that in the series of the metals able to form borides with UB,2-type structure the metallic radius of Zr corresponds to the lowest limit (t = 1.60 X 10 pm). 

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