Anions ionic radius

In AB -type structures, there are two types of atoms, ions, or molecules. The larger spheres are usually visualized to form an A-type lattice, and the smaller ones occupy some fraction of the holes (cubic, octahedral, or tetrahedral) in that lattice. Which holes are occupied is predicted by the radius ratio of the two spheres. For ionic crystals, the radius ratio is usually calculated as the radius of the cation over that of the anion. Ionic radii are derived from high-resolution X-ray studies. 

Relative stability of crystal lattices The hard-sphere model introduced by Born and Madelung was later used to understand the relative stability of crystal structures. Some structures, typical of binary oxides, are represented in Fig. 1.1. An important parameter is the ratio r+/r between the cation and the anion ionic radii (r+ and r, respectively). When cations... 

Ionic radius is the radius of a cation or an anion. Ionic radius affects the physieal and ehemical properties of an ionic compound. For example, the three-dimensional stmeture of an ionie eompound depends on the relative sizes of its eations and anions. 

Cation Ionic Radius (nm) Anion Ionic Radius (nm)... 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

The pentahalides of phosphorus, PX, in the gas phase exhibit varying tendencies to dissociate into trihaUde and halogen. InstabiUty increases with increasing ionic radius of the halogen. The pentafluoride appears to be thermally stable. Dissociation of the pentachloride, a few percent at 100°C and 101.3 kPa (1 atm), is essentially completed at 300°C (36). The pentabromide is partially dissociated in the Hquid state and totally dissociated above ca 35°C (39). Pentaiodide does not exist. The molecules of PF and PCl in the vapor phase are trigonal bipyramids. In the crystalline state, both pentachloride and pentabromide have ionic stmctures, ie, [PClJ IPClg] and [PBr4]" PBrJ , respectively. The PX" 4 cations are tetrahedral and the PX anion is octahedral (36,37). 

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. 

Since hydrofluoride synthesis is based on thermal treatment at relatively high temperatures, the possibility of obtaining certain fluorotantalates can be predicted according to thermal stability of the compounds. In the case of compounds whose crystal structure is made up of an octahedral complex of ions, the most important parameter is the anion-cation ratio. Therefore, it is very important to take in to account the ionic radius of the second cation in relation to the ionic radius of tantalum. Large cations, are not included in the... 

The ionic radius of an element is its share of the distance between neighboring ions in an ionic solid (12). The distance between the centers of a neighboring cation and anion is the sum of the two ionic radii. In practice, we take the radius of the oxide ion to he 140. pm and calculate the radii of other ions on the basis of that value. For example, because the distance between the centers of neighboring Mg2+ and O2 ions in magnesium oxide is 212 pm, the radius of the Mg21 ion is reported as 212 pm - 140 pm = 72 pm. 

In an ionic solid, the coordination number means the number of ions of opposite charge immediately surrounding a specific ion. In the rock-salt structure, the coordination numbers of the cations and the anions are both 6, and the structure overall is described as having (6,6)-coordination. In this notation, the first number is the cation coordination number and the second is that of the anion. The rock-salt structure is found for a number of other minerals having ions of the same charge number, including KBr, Rbl, MgO, CaO, and AgCl. It is common whenever the cations and anions have very different radii, in which case the smaller cations can fit into the octahedral holes in a face-centered cubic array of anions. The radius ratio, p (rho), which is defined as... 

In such systems as (M, Mj (i/2))X (M, monovalent cation Mj, divalent cation X, common anion), the much stronger interaction of M2 with X leads to restricted internal mobility of Mi. This is called the tranquilization effect by M2 on the internal mobility of Mi. This effect is clear when Mj is a divalent or trivalent cation. However, it also occurs in binary alkali systems such as (Na, K)OH. The isotherms belong to type II (Fig. 2) % decreases with increasing concentration of Na. Since the ionic radius of OH-is as small as F", the Coulombic attraction of Na-OH is considerably stronger than that of K-OH. 

When three different kinds of spherical ions are present, their relative sizes are also an important factor that controls the stability of a structure. The PbFCl type is an example having anions packed with different densities according to their sizes. As shown in Fig. 7.5, the Cl- ions form a layer with a square pattern. On top of that there is a layer of F ions, also with a square pattern, but rotated through 45°. The F ions are situated above the edges of the squares of the Cl- layer (dotted line in Fig. 7.5). With this arrangement the F -F distances are smaller by a factor of 0.707 (= /2) than the CP-CP distances this matches the ionic radius ratio of rF-/rcl- = 0.73. An F layer contains twice as many ions as a CP layer. Every Pb2+ ion is located in an antiprism having as vertices four F and four... 

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. 

It is apparent from the data shown in Table 7.4 that there is a great difference between the sizes of some ions. For example, the ionic radius of Li+ is 60 pm, whereas that of Cs+ is 169 pm. When these ions are forming a crystal with Cl, which has a radius of 181 pm, it is easy to understand that the geometrical arrangement of ions in the crystals may be different even though both LiCl and CsCl have equal numbers of cations and anions in the formulas. 

Figure 9.3 shows the relationship between ionic radius and proton affinity in a graphical way for monatomic ions having a — 1 charge. It is clear that to a good approximation there is a correlation between the size of the anion and its proton affinity. While this is in no way a detailed study, it is clear that the smaller (and thus harder] the negative ion (with the same type of structure) the more strongly it binds a proton. 

Compound Ionic radius of cation (pm) Ionic radius of anion (pm) Lattice energy (kj/mole)... 

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

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...

---