Main-group elements ionic radii

The radii of cations and anions derived from atoms of the main-group elements are shown at the bottom of Figure 6.13. The trends referred to previously for atomic radii are dearly visible with ionic radius as well. Notice, for example, that ionic radius increases moving down a group in the periodic table. Moreover the radii of both cations (left) and anions (right) decrease from left to right across a period. 

Ionic radius The radius assigned to a monatomic ion, 154 main-group elements, 153t Ionic solids, 240-245 Ionization expression, 378q percent, 362... 

Ionic radius data from R. D. Shannon, Acta Crystallogr. 1976, A32, 751. Note how the ionic radii are smaller than the covalent radii the ionic radii are most suitable for bonds where the electronegativity difference between the constituent atoms is large (see Scheme 8.1). These radii should be combined with ionic radii for main group elements to predict bond distances. 

To understand how you compute ionic radii, consider the determination of the radius of an 1 ion in a Uthium iodide (LU) crystal. Figure 9.7 depicts a layer of ions in Lil. The distance between adjacent iodine nuclei equals twice the 1 radius. From x-ray diffraction experiments, the iodine—iodine distance is found to be 426 pm (1 pm = 1 X 10 m). Therefore, Ihe 1 radius in Lil is 5 X 426 pm = 213 pm. Other crystals give approximately the same radius for Ihe 1 ion. Table 9.3, which Usts average values of ionic radii obtained from many compounds of the main-group elements, gives 216 pm for the 1 radius. 

Figure 7.12 shows the ionic radii for those ions of main group elements that are isoelectronic with noble gases and compares them to the radii of the parent atoms. Note that the ionic radius, like the atomic radius, increases from top to bottom in a group. 

In the 1939 edition of his classic book The Nature of the Chemical Bond, Pauling listed an extensive table of ionic radii of main-group elements obtained by fixing the radius ratios between the four isoelectronic ion pairs and Na", CP and K", Br and Rb", and I and Cs" as the inverse ratio between the effective nuclear charges r+/r = [37]. The effective nuclear charge of an atom or ion is... 

Many attempts to interprete the cesium effect proceed from the special position of cesium in the periodic system of the elements and in the first main group Cesium forms the cation with the largest ionic radius and follows the Tl ion with respect to the largest polarizability (Table 10). 

The physical properties of metal alkoxides depend primarily on the characteistics of the metal (e.g., the electronegativity, valence, atomic radius, and coordination number) and secondarily on the characteristics of the alkyl group (e.g., the size and shape). There is a change from the solid, nonvolatile ionic alkoxides of some of the alkali metals to the volatile covalent liquids of elements of valence 3, 4, 5, or 6 (e.g., Al, Si, Ti, Zr, Sb, and Te), whereas alkoxides of metals with intermediate electronegativities, such as La and Y, are mainly solids (Table 5.1). 

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