Packing of ions

Fig. 5.8. (a) Packing of the unequally sized ions of sodium chloride to give a f.c.c. structure KCl and MgO pack in the same v/ay. (b) Packing of ions in uranium dioxide this is more complicated than in NaCi because the U and O ions are not in 1 1 ratio. 

In conjugated molecules one or other of the possible protonation sites may be more or less favoured by solvation effects and for this reason sites of protonation are often solvent dependent. In some instances, similar stability of two possible cations results in tautomeric equilibria and these too may be solvent dependent. Just as solute-solvent interactions have an effect on the relative stability of two possible cations formed from a conjugated molecule, so in solid salts stability relationships depend on the mode of packing of ions, which determines interactions with the nearest neighbours. Therefore the types of cation observed in solid salts are not necessarily the most stable ones in solution. 

Fuchigami Y. Hydrolysis of methyl acetate in distillation column packed with reactive packing of ion exchange resin. J Chem Eng Japan 1990 23 354-358. 

The valence state of ions listed in Table4.2.2 are obtained from the oxidation states of the atoms in the compounds. They are only formal values and they do not indicate the number of transferred electrons. In other words, the bonding type between the atoms is not considered. These radii are effective ionic radii and they can be used for rough estimation of the packing of ions in crystals and other calculations. 

Fig. 2.1 Packing of ions (a) simple cubic packing showing an interstice with eightfold coordination (b) hexagonal close packing (c) cubic close packing showing a face-centred cubic cell.

In an ionic crystal the structure adopted represents the most efficient packing of ions of opposite charge subject to the additional influence of thermal energy. Electrostatic forces are non-directional and the structure adopted is determined by three main controls (i) the relative numbers of cations and anions, (ii) the relative sizes of positive and negative ions and (iii) ionic shapes. Additional species, including hydrated or otherwise coordinated ions together with molecules of water... 

The growth rates of the a and phases in KNOg have been measured as a function of the number of transformations and the temperature during the a -> /3 and /5 -> a transitions. It was found that the transformation rate is influenced by recrystallization and relaxation processes. Dielectric transitions in MNOg (M = Li, Na, K, Rb, Cs, Ag, Tl, or NH4) have characteristic temperatures which depend on the degree of packing of ions in the crystal lattice. The same factor also influences the melting points of the compounds. 

Closest Packing of Ions 31 Arrangement (b) makes the most efficient use of space. 

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