Covalent-ionic bond character

It is customary to use the line between two atomic symbols, A—B, to represent a normal covalent bond, with the usual amount of ionic character. In the discussion that follows, A B is used to represent a pure covalent single bond, and A+B to represent the ionic structure that is hybridized with A B to give A—B. A pure covalent double bond is represented by A=B. Thus in the molecule NF3 we might describe each NF bond as involving N+ F , N F , and N F +, that is, as having some covalent double-bond character. 

Transitions between other extreme types of bonds (covalent to metallic covalent to ion-dipole, etc.) can also occur without discontinuity, and the bonds of intermediate character can be discussed in terms of resonance between structures of extreme type in the same way as for covalent-ionic bonds. 

From the preceding, it might be supposed that covalent character in predominantly ionic compounds always destabilizes the compound. This is not so. Instability results from polarization of the anion causing it to split into a more stable compound (in the above cases the oxides) with the release of gaseous acidic anhydrides. As will be seen in Chapter 16, many very stable, very hard minerals have covalent-ionic bonding. 

We see again that there is but one principle which causes a chemical bond between two atoms all chemical bonds form because electrons are placed simultaneously near two positive nuclei. The term covalent bond indicates that the most stable distribution of the electrons (as far as energy is concerned) is symmetrical between the two atoms. When the bonding electrons are somewhat closer to one of the atoms than the other, the bond is said to have ionic character. The term ionic bond indicates the electrons are displaced so much toward one atom that it is a good approximation to represent the bonded... 

Needless to say, if ionic character affects the energy stability of a chemical bond it also affects the chemistry of that bond. The tendency toward minimum energy is one of the factors that determine what chemical changes will occur. As a bond becomes stronger, more energy is required to break that bond to form another compound. Hence we see that ionic bonds are favored over covalent bonds and that ionic character in a bond affects its chemistry. 

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... 

Although the S—O bond lengths in sulphoxides and sulphones seem to indicate that these are covalent double bonds, the dipole moments of these compounds obtained by calculation or by experimental measurements support the semipolar single-bond character in these compounds, and they should be represented as S - O, with about 66% ionic character14,15. 

All ionic bonds have some covalent character. To see how covalent character can arise, consider a monatomic anion (such as Cl-) next to a cation (such as Na+). As the cation s positive charge pulls on the anion s electrons, the spherical electron... 

Ions stack together in the regular crystalline structure corresponding to lowest energy. The structure adopted depends on the radius ratio of cation and anion. Covalent character in an ionic bond itnposes a directional character on the bonding. 

Ionic bonds may be fully as strong as covalent bonds, so that properties such as hardness, solubility, melting point, ionization in solution, and chemical character are not especially valuable criteria as a rule. Sometimes comparison of properties with those of compounds of known bond type permits reasonably certain conclusions to be drawn. Thus the similarity in physical properties as well as in atomic arrangement of SiC, AIN, and diamond suggests that all three substances contain covalent bonds. PbS is like FeS2, MoS2, etc. in properties rather than like CaS, so that it is improbable that PbS is an ionic substance. 

Equation 1-1 with Ax = 1 for N—F leads to 22.1% ionic character and bond moment 1.46 D, a little above the straight line in Figure 1-3. Let us assume that the contribution of the pure covalent structure 1 has the value of 47.2%, calculated from the value 77.9% for each bond (22.1% ionic character). Since the three structures of type 2 contribute 3 X 2.74% = 8.2%, the structures of type 3 contribute the remainder, 44.6%. This value leads to 14.9% for the amount of double-bond character of each of the bonds in the NF3 molecule, close to the value 15% for CHF3, CC1F3, and C1F3 calculated from the shortening of the bond length,59 which is by 0.05 A. 

When the difference in electronegativities is great, the orbital may be so far over to one side that it barely covers the other nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion, leaving us with basically only one type of bond in organic molecules. Most bonds can be considered intermediate between ionic and covalent. We speak of percent ionic character of a bond, which indicates the extent of electron-cloud distortion. There is a continuous gradation from ionic to covalent bonds. 

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