Change in Bond Type

8 The squares of the coefficients of terms in a composite wave function are interpreted as representing the magnitudes of the contributions of the corresponding structures. 

For a molecule such as hydrogen chloride we write the two reasonable 

Instead of using this description of the bond as involving resonance between an extreme covalent bond H C1 and an extreme ionic bond H+Cl-, we may describe the bond as a covalent bond with partial ionic character, and make use of the valence line, writing H—Cl (or H—Cl ) 

The amount of ionic character of a bond in a molecule must not be confused with the tendency of the molecule to ionize in a suitable solvent. The ionic character of the bond is determined by the importance of the ionic structure (A+B ) when the nuclei are at their equilibrium distance (1.275 A for HC1, for example), whereas the tendency to ionize in solution is determined by the relative stability of the actual molecules in the solution and the separated ions in the solution. It is reasonable, however, for the tendency toward ionization in solution to accompany large ionic character of bonds in general, since both result from great difference in electronegativity of the bonded atoms.4 

4 A discussion of the ionization of the hydro halo genic acids in aqueous solution is given in App. XI. 

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An abrupt change in properties in a series of compounds, as in the melting points or boiling points of halides, is often taken as indicating an abrupt change in bond type. Thus of the fluorides... 

Discontinuous Change in Bond Type.5—In molecules and complex ions of certain types continuous transition from one extreme bond type to another is not possible. In order for continuous transition to be possible between two extreme bond types the conditions for resonance between the corresponding structures must be satisfied. The most important of these conditions is that the two structures must involve the same numbers of unpaired electrons. If the two structures under consideration involve different numbers of unpaired electrons, then the transition between the two must be discontinuous, the discontinuity being associated with the pairing or unpairing of electrons ... 

The great change between aluminum trifluoride and silicon tetrafluoride is not due to any great change in bond type—the bonds are in all cases intermediate in character between extreme ionic bonds... 

Polymorphic changes involving change in bond type... 

The combination reactions involve a change in bond type from ionic to covalent and the dissociation reaction the reverse ... 

Figure 9.23 Properties of the Period 3 chlorides. Samples of the compounds formed from each of the Period 3 elements with chlorine are shown in periodic table sequence in the photo. Note the trend in properties displayed in the bar graphs as AEN decreases, both melting point and electrical conductivity (at the melting point) decrease. These trends are consistent with a change in bond type from ionic through polar covalent to nonpolar covalent.

Magnetometric measurements were carried out with a micromethod by Theorell (75,77). The change in spectrum and color with pK 10.9 is accompanied by a change in bond type from essentially ionic bonds, Xmoi =... 

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