Ionic crystals bonding

FIGURE 1 4 An ionic bond IS the force of attrac tion between oppositely charged ions Each Na ion (yellow) in the crystal lattice of solid NaCI IS involved in ionic bonding to each of six surrounding Cl ions (green) and vice versa... 

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

The formulated principals correlating crystal structure features with the X Nb(Ta) ratio do not take into account the impact of the second cation. Nevertheless, substitution of a second cation in compounds of similar types can change the character of the bonds within complex ions. Specifically, the decrease in the ionic radius of the second (outer-sphere) cation leads not only to a decrease in its coordination number but also to a decrease in the ionic bond component of the complex [277]. 

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... 

Of the three principal classes of crystals, ionic crystals, crystals containing electron-pair bonds (covalent crystals), and metallic crystals, we feel that a good understanding of the first class has resulted from the work done in the last few years. Interionic distances can be reliably predicted with the aid of the tables of ionic radii obtained by Goldschmidt1) by the analysis of the empirical data and by Pauling2) by a treatment based on modem theories of atomic structure. The stability,... 

In the past, ionic radii have often been compared with observed interatomic distances without much regard to the nature of the crystal from which they were derived. Recently several investigators19 have concluded that in many crystals the bond between atoms does not consist of the electrostatic attraction of only slightly deformed ions. Goldschmidt in particular has divided crystals into two classes, ionic and atomic crystals, and has shown that ionic radii (using Wasastjema s set) do not account for the observed inter-atomic distances in atomic crystals. In the following pages our crystal radii will be compared with the experimental dis-... 

A question which has been keenly argued for a number of years is the following if it were possible continuously to vary one or more of the parameters determining the nature of a system such as a molecule or a crystal, say the effective nuclear charges, then would the transition from one extreme bond type to another take place continuously, or would it show discontinuities For example, are there possible all intermediate bond types between the pure ionic bond and the pure electron-pair bond With the development of our knowledge of the nature of the chemical bond it has become evident that this question and others like it cannot be answered categorically. It is necessary to define the terms used and to indicate the point of view adopted and then it may turn out, as with this question, that no statement of universal application can be made. 

Reactions of UCI4 with [Li RC(NCy)2 (THF)]2 (R = Me, Bu ) in THF gave the tris(amidinate) compounds [RC(NCy)2]3UCl that could be reduced with lithium powder in THF to the dark-green homoleptic uranium(lll) complexes [RC(NCy)2]3U. Comparison of the crystal structure of [MeC(NCy)2]3U with those of the lanthanide analog showed that the average U-N distance is shorter than expected from a purely ionic bonding model. ... 

When an ionic compound dissolves in water, energy is needed to break the ionic bonds of the crystal. As the ions attach to the water molecules and become hydrated, energy is released. The process is endothermic if the energy needed to break the bonds is greater than the energy released when the ions attach to water. 

Packing in / Im 2 H20 also shows some distinct features that may be related to the existence of the ionic species in the crystal. Hydrogen bonding is, of course, a primary feature (Fig. 43). An extensive network exists in this crystal which has the form of endless chains rather than that of loops usually found for the similarly double-faced (H-bond donor and acceptor) alcohols (cf. Fig. 19). As already mentioned, the carboxylate function has four connections, while its neutral —COOH neighbor maintains three H-bond contacts. The inner water molecule with respect... 

There has never been a really clear understanding of what a bond line stands for. Originally it was meant to indicate simply that the two atoms between which it is drawn are held strongly together. However, it is now usually taken to indicate a shared pair of electrons, that is, a covalent bond. In contrast, the presence of ionic bonds in a molecule or crystal is usually implied by the indication of the charges on the atoms, and no bond line is drawn. This immediately raises the question of how polar a bond has to be before the bond line is omitted. Whereas the structure of the LiF molecule would normally be written as Li+F without a bond line, even the highly ionic BeF2 is often written as F—Be—F rather than as F Be2+ F . 

Ionic radii are quoted in Tables 2.3 and 2.5 for a large number of cations including those of the elements in groups 13, 14, 15, and 16, which do not form predominately ionic bonds. These values were obtained by subtracting the fluoride or oxide ion radius obtained from predominantly ionic solids from the length of a bond that is not predominantly ionic. The very small values for the radii of cations obtained in this way do not bear much relation to the real size of the atom in the crystal or molecule. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

Ionically bonded crystals contain both long-range and short-range bonding forces because like ions repel each other, while unlike ones attract. 

Physical hardness can be defined to be proportional, and sometimes equal, to the chemical hardness (Parr and Yang, 1989). The relationship between the two types of hardness depends on the type of chemical bonding. For simple metals, where the bonding is nonlocal, the bulk modulus is proportional to the chemical hardness density. The same is true for non-local ionic bonding. However, for covalent crystals, where the bonding is local, the bulk moduli may be less appropriate measures of stability than the octahedral shear moduli. In this case, it is also found that the indentation hardness—and therefore the Mohs scratch hardness—are monotonic functions of the chemical hardness density. 

The principal intention of the present book is to connect mechanical hardness numbers with the physics of chemical bonds in simple, but definite (quantitative) ways. This has not been done very effectively in the past because the atomic processes involved had not been fully identified. In some cases, where the atomic structures are complex, this is still true, but the author believes that the simpler prototype cases are now understood. However, the mechanisms change from one type of chemical bonding to another. Therefore, metals, covalent crystals, ionic crystals, and molecular crystals must be considered separately. There is no universal chemical mechanism that determines mechanical hardness. 

In general, overlap of incompletely filled p orbitals results in large deviations from pure ionic bonding, and covalent interactions result. Incompletely filled / orbitals are usually well shielded from the crystal field and behave as essentially spherical orbitals. Incompletely filled d orbitals, on the other hand, have a large effect on the energetics of transition metal compounds and here the so-called crystal field effects become important. 

The Na+ and Cl- ions can be considered as negatively and positively charged spheres that attract each other. Since positive (+) and negative (-) charges form an electric field in all directions, the electrostatic force of attraction (ionic bond) is not just in one direction. In the NaCl crystal, each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions (Figure 2). Because of this, the structure of NaCl is not a molecule but it is in the form of an ionic crystal in which many ions are found together. 

Formation of the ionic bond between sodium and chloride ions in the NaCl crystal. 

---