The Lengths of Polar Bonds

2 shows that the ions have almost perfectly spherical charge clouds, indicating that the crystal is very nearly fully ionic, as we will discuss in detail in Chapter 6. In the next section we discuss how the lengths of bonds are affected by their polarity. 

Calculated from the Schomaker-Sievenson equation. bSum of the covalent radii (Table 2.1). 

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The concept of back-bonding is not necessary to account for the lengths of polar bonds that are shorter than the sum of the covalent radii. These bonds are short because of the attraction between the atoms due to their opposite charges. 

Back-bonding has usually been discussed in terms of the orbital model (Chapter 3), and we will revisit it again in later chapters. For the moment we need only emphasize that since the apparently short bond lengths can be accounted for in terms of the polarity of the bonds. Bond lengths do not provide any compelling evidence for the concept of back-bonding. 

When the electrons in a covalent bond are shared equally, the length of the bond between the atoms can be approximated as the sum of the covalent radii. However, when the bond is polar, the bond is not only stronger than if it were purely covalent, it is also shorter. As shown earlier, the amount by which a polar bond between two atoms is stronger than if it were purely covalent is related to the difference in electronegativity between the two atoms. It follows that the amount by which the bond is shorter than the sum of the covalent radii should also be related to the difference in electronegativity. An equation that expresses the bond length in terms of atomic radii and the difference in electronegativity is the Schomaker-Stevenson equation. That equation can be written as... 

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