Gaseous alkali metal halides ionic bonds

Gaseous alkali metal halides ionic bonds 

The alkali metal halides, MX, fonn solids at room temperature. The best known member of the family, NaCl, melts at 801°, the vapor pressure reaches 1 torr at 865°, and the melt boils at 1413°. The dominant species in gaseous alkali metal halides is the monomeric formula unit, MX(g), but smaller amounts of dimers (M2X2), trimers (M3X3) and tetramers (M4X4) have also been detected by mass spectroscopy. Information about the monomeric molecules obtained by spectroscopic studies at high temperatures is collected in Table 5.1 [1]. 

Solid and molten alkali metal halides are described as consisting of M+ and X ions. How should we describe the molecular units in the gas phase Do they consist of a pair of ions, M+X , which are held together by Coulomb attraction  

When the model is refined to include polarization effects, i.e. to allow for that fact the electric field set up by each ion in the monomer deforms the electron cloud in the other, calculated molecular dipole moments are in significantly better agreement with the observed values. Polarization effects do not, however, lead to significant improvement of the fit between calculated and experimental dissociation energies. 

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Gaseous alkali metal halides ionic bonds... 

GASEOUS ALKALI METAL HALIDES IONIC BONDS... 

This leads to a reconsideration of the concept contact or intimate ion pair. Although these terms have been in use for about 15 years, the term "unionized species appears more appropriate 97). For instance, the bond in a diatomic alkali metal halide as is known to exist in the gas phase is certainly not purely ionic (102) and its formation from gaseous ions can be described by the functional approach (97). 

The simplest systems containing ionic bonds are the gaseous molecules of alkali halides and oxides, the structures of which are noted in Chapters 9 and 12 we refer later to the halide molecules in connection with polarization. The importance of the ionic bond lies in the fact that it is responsible for the existence at ordinary temperatures, as stable solids, of numerous metallic oxides and halides (both simple and complex), of some sulphides and nitrides, and also of the very numerous crystalline compounds containing complex ions, particularly oxy-ions, which may be finite (CO3 , NO3, SOl", etc.) or infinite in one, two, or three dimensions. 

Another approach calculates the percent ionic character of a bond by observing the actual behavior of a polar molecule in an electric field. A value of 50% ionic character divides substances we call ionic from those we call covalent. Such methods show 43% ionic character for the H—F bond and expected decreases for the other hydrogen halides H—Cl is 19% ionic, H—Br 11%, and H—I 4%. A plot of percent ionic character vs. AEN for a variety of gaseous diatomic molecules is shown in Figure 9.22C. The specific values are not important, but note that percent ionic character generally increases with AEN. Another point to note is that whereas some molecules, such as Cl2(g), have 0% ionic character, none has 100% ionic character. Thus, electron sharing occurs to some extent in every bond, even one between an alkali metal and a halogen. 

Table 6.1. Gaseous heteronuclear diatomic molecules AB(g) hydrogen halides HX interhalogen compounds XK alkali metal hydrides MH, and inter-alkali-metal compounds MM. Electric dipole moments, ionic characters, qjc equilibrium bond distances. Re, vibrational wavenumbers, < dissociation energies at zero K, Dq rednced masses of the predominant isotopomers, /tm and force constants,. ...

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