Electronegativity difference bond character determination

Figure 8.21 This graph shows that the difference in electronegativity between bonding atoms determines the percent ionic character of the bond. Above 50% ionic character, bonds are mostly ionic. 

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... 

But the nature of the compounds formed when the metal and nonmetal are closer to the center of the periodic table is less obvious. Their electronegativities are closer together. And the electronegativity difference between the atoms in a compound determines the nature of the bond. Recall that differences of 1.7 or more result in ionic bonds atoms with differences less than 1.7 form bonds with some covalent character. Lead sulfide (PbS) is an example of such a compound. Lead has an electronegativity of 1.9 sulfur is 2.5. The difference of 0.6 is less than 1.7, so the bond between them should have some covalent character. Like sodium chloride, lead sulfide is a crystalline compound. However, it is dark and shiny, quite unlike salt. It is also insoluble in water, which indicates a high degree of covalency in the lead-sulfur bond, just as one would expect. 

I The electronegativity difference determines the character of a bond between atoms. 

To decide which bond is more polar, look up EN values for H, Cl, and O in Figure 10-6, and then compute electronegativity differences, AEN, for H—Cl and H—O bonds. The greater the electronegativity difference, the more polar the bond. To determine the percentage ionic character, we use the curve in Figure 10-7. 

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. 

Let us discuss the importance of charge separation in bonds involving atoms of different electronegativity, for example, C and N, O, or Cl. The extent of partial ionic character in such polar covalent bonds is a key factor in determining a compound s... 

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