Electronegativity percent ionic character

C—M Bond Difference in Electronegativity Percent Ionic Character ... 

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. 

Electronegativity difference Type of bond Percent ionic character... 

Formula Electronegativity difference Percent ionic character Type of bond... 

FIGURE 3.10 The variation in percent ionic character to a bond and the difference in the electronegativities of the atoms. 

Having shown that the weighting coefficient (A) of the term giving the contribution of an ionic structure to the molecular wave function is related to the dipole moment of the molecule, it is logical to expect that equations could be developed that relate the ionic character of a bond to the electronegativities of the atoms. Two such equations that give the percent ionic character of the bond in terms of the electronegativities of the atoms are... 

Although the equations look very different, the calculated values for the percent ionic character are approximately equal for many types of bonds. If the difference in electronegativity is 1.0, Eq. (3.70) predicts 19.5% ionic character while Eq. (3.71) gives a value of 18%. This difference is insignificant for most purposes. After one of these equations is used to estimate the percent ionic character, Eq. (3.61) can be used to determine the coefficient A in the molecular wave function. Figure 3.10 shows how percent ionic character varies with the difference in electronegativity. 

What Pauling electronegativity is predicted for an element X if the ff-Xbond energy is 402 kj mol-1 The ff-ff bond energy is 432 kj mol-1 and the X-X bond energy is 335 kj mol-1. What would be the percent ionic character of the H-X bond If the molecular wave function is written as... 

The percent ionic character of a bond is based on the difference in electronegativity of its constituent atoms and Figure 11.7. 

You can use electronegativity differences to think of chemical bonds as having a percent ionic or a percent covalent character. The graph below plots percent ionic character versus AEN for a number of gaseous binary molecules. Use this graph to answer the questions on the next page. 

Percent Ionic Character Based on Difference in Electronegativity... 

According to Equation 3-15 bonds between atoms with electronegativity difference 1.7 have 50 percent ionic character and 50 percent covalent character. Thus bonds between fluorine and any of the metals or of the elements H, B, P, As, Te, with electronegativity near 2, are largely ionic in character, and bonds between oxygen and any of the metals are 50 percent or more ionic. For a molecule such as HF, containing a single bond, we have discussed the bond type in terms of... 

The Alkali Metals.—Bonds of the alkali metals with all nonmetals are essentially ionic (with more than 50 percent ionic character—electronegativity difference greater than 1.7) except for Li—I, Li—C, and Li—S, with about 43 percent ionic character. 

This large amount of electron transfer is not incompatible with the electroneutrality principle. The electronegativity of aluminum is 1.5, and that of gold is 2.4. The difference corresponds to 18 percent ionic character of the Au—A1 bonds, which with valence 6.60 for gold would lead to the charge —1.19 on the gold atom. To restore it to neutrality 1.19 electrons would have to be transferred to two aluminum atoms. 

Table 3.2 shows how you can think of bonds as having a percent ionic character or percent covalent character, based on their electronegativity differences. When bonds have nearly 50% ionic or covalent character, they have characteristics of both types of bonding. 

Application of this definition to various compounds (in the gas phase) gives the results shown in Fig. 13.12, where percent ionic character is plotted versus the difference in the electronegativity values of X and Y. Note from this plot that ionic character increases with electronegativity difference, as expected. However, none of the bonds reaches 100% ionic character, even though compounds with the maximum possible electronegativity differences are considered. Thus, according to this definition, no individual bonds are completely ionic. This conclusion is in contrast to the usual classification of many of these compounds (as solids). All the compounds shown in Fig. 13.12 with more than 50% ionic character are normally considered to be ionic solids. Recall, however, that the results in Fig. 13.12 are for the gas phase, where in-... 

This graph shows how the percent ionic character of a bond depends on the difference in electronegativity of the atoms that form it. Above 50% ionic character, bonds are mostly ionic. What is the percent ionic character of a pure covalent bond ... 

The percent ionic character of a bond can be approximated by the formula 16A + 3.5A, where A is the magnitude of the difference in the electronegativities of the atoms (see Fig. 3.7). Calculate the percent ionic character of HF, FdCl, FdBr, Fdl, and CsF, and compare the results with those in Table 3.7. 

There is no sharp distinction between a polar bond and an ionic bond, but the following rule is helpful in distinguishing between them. An ionic bond forms when the electronegativity difference between the two bonding atoms is 2.0 or more. This rule applies to most but not all ionic compounds. Sometimes chemists use the quantity percent ionic character to describe the nature of a bond. A purely ionic bond would have 100 percent ionic character, although no such bond is known, whereas a nonpolar or purely covalent bond has 0 percent ionic character. 

Application of this definition to various compounds (in the gas phase) gives the results shown in Fig. 8.13, where percent ionic character is plotted versus the difference in the electronegativity values of X and Y Note from this plot that ionic character increases with electronegativity difference, as expected. However, none of the bonds reaches 100% ionic... 

The degree of ionic character depends on the nature of the metal, the medium, and the substituents on the carbanionic carbon atom. For simple alkyls (those in which resonance stabiHzation of the negative charge is not expected), the nature of the metal is especially important. The percent ionic character (alternatively, the percent contribution of the ionic resonance structure to the hybrid) increases with increasing difference in electronegativity of the two atoms. 

Example 6-3. Using the revised Pauling electronegativities from Table 5.12 and applying Equation (6.2), calculate the percent ionic character for each of the following molecules (a) HE, (b) HI, (c) LiF, and (d) BeClj. 

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