Polar covalent bonds percent ionic character

How well can we tell the difference between an ionic bond and a polar covalent bond The only honest answer to this question is that there are probably no totally ionic bonds between discrete pairs of atoms. The evidence for this statement comes from calculations of the percent ionic character for the bonds of various binary compounds in the gas phase. These calculations are based on comparisons of the measured dipole moments for molecules of the type X—Y with the calculated dipole moments for the completely ionic case, X+Y. We performed a calculation of this type for HF in Section 13.3. The percent ionic character of a bond can be defined as... 

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. 

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. 

How do we know whether a bond between two atoms is ionic or covalent The difference in electronegativity between the two atoms determines the character of the bond formed between them. As the difference in electronegativity increases, the polarity of the bond (or percent ionic character) increases. 

Polar covalent unequal electron sharing Percent ionic character of a bond X—Y... 

Although the designations covalent, polar covalent, and ionic can be useful, sometimes chemists wish to describe and compare chemical bonds with more precision. For this purpose, we can use Equation 8.1 to calculate the dipole moment we would expect if the chaiges on the atoms were discrete instead of partial that is, if an electron had actually been transferred from one atom to the other. Comparing this calculated dipole moment with the measured value gives us a quantitative way to describe the nature of a bond rising the term percent ionic character, which is defined as the ratio of observed p, to calculated p, multiplied by 100. 

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