Halides ionic radii

Figure 19-4 contrasts the effective sizes of the halide ions. Each of these dimensions is obtained from the examination of crystal structures of many salts involving the particular halide ion. The effective size found for a given halide ion is called its ionic radius. These radii are larger than the covalent radii but close to the van der Waals radii of neutral atoms. 

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

The linear dependence of the pitting potential on ionic radius is likely a reflection of the similarly linear relationship between the latter and the free energy of formation of aluminum halides.108 It is reasonable to assume that the energy of adsorption of a halide on the oxide is also related to the latter. Hence, one could postulate that the potential at which active dissolution takes place is the potential at which the energy of adsorption overcomes the energy of coulombic repulsion so that the anions get adsorbed. 

The ionic radii of the commonest oxidation states are presented in Table 2. There is evidence of an actinide contraction of ionic radii as the 5/ orbitals are filled and this echoes the well established lanthanide contraction of ionic radii as the 4/orbitals are filled. Actinides and lanthanides in the same oxidation state have similar ionic radii and these similarities in radii are obviously paralleled by similarities in chemical behaviour in those cases where the ionic radius is relevant, such as the thermodynamic properties observed for halide hydrolysis. 

Fig. 12.2 The ratio of radii, k (=ionic radius/covalent radius), for alkali metal eations (M ) and halide anions (X ) in aqueous solutions (Eqs. 12.6a, b). In the right angled triangle, ABC, E and D are the mid points of AB and AC...

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

The concept that the ionic radius is relatively independent of the structure of the solid arose intuitively from experimental observations carried out on alkali halides, which are ionic solids par excellence. Figure 1.3 shows the evolution of interatomic distances in alkali halides as a function of the types of anion and cation, respectively. Significant parallelism within each of the two families of curves may be noted. This parallelism intuitively generates the concept of constancy of the ionic radius. 

Table 6.2 shows the detachment energy of one water molecule from a hydrated halide ion cluster [41]. The strength of the water-halide interactions is reduced as the ionic radius increases in the order of Fspecific adsorption in an electrochemical environment. It is clear that the nonspecific adsorption behavior of F is due to its strongly bound solvation shell. Due to... 

J6 The ammonium km is about the same size (r+ = 151 pm) as the potassium ion ir. 152 pm) and this is a usef ul fact to remember when explaining the resemblance in properties between these two tuns. For example, (he solubilities of ammonium salts arc similar to those of potassium sails. Explain the relation between ionic radius and soloWiiy. On the other hand, all of the potassium halides crystallize in the NaClstrocture with C.N. = 6 (see Chapter 4). but none of the ammonium halides does so. The coordination numbers of the ammonium halides are either four or eight- Suggest an explanation. 

The azide ion has an ionic radius of 148 pm and forms many ionic and covalent compounds that are similar to those of the halides, (a) Write the Lewis formula for the azide ion and predict the N—N—N bond angle, (b) On the basis of its ionic radius, where in Group 17 would you place the azide ion ... 

Each rubidium halide (Group VIIA element) crystallizing in the NaCl-type lattice has a unit cell length 30 pm greater than that for the corresponding potassium salt of the same halogen. What is the ionic radius of Rb+ computed from these data ... 

A second alternative which accords with first order kinetics consists in the formation of a low steady-state concentration of dissociated ions, followed by rate-determining attack of halide on the quasiphosphonium ion (k2<Fuoss equation which permits calculation of K from the mean ionic radius of the ions and the dielectric constant of the medium (7). For the present purpose we... 

These solvent effects can be explained in classical terms. The intrinsic (gas-phase) reactivity increases as the as the ionic radius falls. The reasons are simple as the anion becomes smaller, the surplus electron generates greater interelectronic repulsions and the reactivity rises. Dipolar aprotic solvents interact only weakly with the halides, so they do not appreciably affect this reactivity order. In protic solvents where hydrogen bonds play an important role, the smaller the ion, the more stabilized it becomes and the reactivity order is inverted. 

The idea of the existence of specific adsorption appeared as an explanation for the fact that electrocapillary curves at mercury electrodes are different for different electrolytes at the same concentration (Fig. 3.4a). For sodium and potassium halides in water the differences arise at potentials positive of fsz, which suggests an interaction with the anions. As the effect is larger the smaller the ionic radius of the anion, the idea of specific adsorption with partial or total loss of hydration arose. 

The interionic distance in lithium fluoride is found to be 2.01 A subtracting from this figure the ionic radius of F" (1.36 A obtained above), one may fix the ionic radius of Li+ as about 0.65 A. However, if estimates of the interionic distances in the remaining lithium halides are made by adding 0.65 A to the respective halide radii, the sums so obtained are about 10 percent less than the observed values (Table 12—1). Similarly... 

As the coordination number of an ion is thus increased from 6 to 8, it appears that the ionic radius also suffers a slight increase, presumably because the repulsion forces, exerted by the electron clouds of neighboring ions, increase. For the rubidium halides and the ammonium halides which can assume either structure (depending upon pressure and temperature), the interionic distances in the structures having coordination number 8 are about 3 percent greater than the distances in structures with coordination number 6. This increase is close to the value that would be... 

The ionic radii for the commonest oxidation states (Table 20-1) are compared with those of the lanthanides in Fig. 20-1. There is clearly an actinide contraction, and the similarities in radii of both series correspond to similarities in their chemical behavior for properties that depend on the ionic radius, such as hydrolysis of halides. It is also generally the case that similar compounds in the same oxidation state have similar crystal structures that differ only metrically. 

Fig. 2.44. Entropy of hydration against reciprocal of ionic radius for halide ions (1 A = 100 pm 1 e.u. = 4.184 J mof ). (Reprinted from J. O M. Bockris and P. P. S. Saluja, J. Phys. Chem. 76 2298, 1972.)...

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