The Partial Ionic Character of Covalent Bonds

Ion Radius Ion Radius Ion Radius Ion Radius Ion Radius Ion Radius 

Often a decision must be made as to whether a molecule is to be considered as containing an ionic bond or a covalent bond. There is no question about a salt of a strong metal and a strong nonmetal an ionic structure is to be written for it. Thus for lithium chloride we write 

Similarly there is no doubt about nitrogen trichloride, NCI3, an oily molecular substance composed of two nonmetals. Its molecules have the covalent structure 

Between LiCl and NClj there are the three compounds BeCl2, BCI3, and CCI4. Where does the change from an ionic structure to a covalent structure occur  

The answer to this question is provided by the theory of resonance. The transition from an ionic bond to a normal covalent bond in a series of compounds such as those mentioned in the preceding sentence does not occur sharply, but gradually. 

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The Partial Ionic Character of Covalent Bonds and the Relative Electronegativity of Atoms... 

As we ve just seen, if you ask Is an X—Y bond ionic or covalent the answer in almost every case is Both, partially A better question is 7b what extent is the bond ionic or covalent The partial ionic character of a bond is related directly to the electronegativity difference (AEN), the difference between the EN values of the bonded atoms a greater AEN results in larger partial charges and a higher partial ionic character. Consider these three chlorine-containing molecules AEN for LiCl(g) is 3.0 - 1.0 = 2.0 for HCl(g), it is 3.0 - 2.1 = 0.9 and for CElg), it is 3.0 - 3.0 = 0. Thus, the bond in LiCl has more ionic character than the H—Cl bond, which has more than the Cl—Cl bond. 

The previous literature on the effects of partial covalence on interatomic distances is contradictory. Pauling (1960) cites the examples of CuF, BeO, AIN, and SiC where observed bond lengths are shorter than the sum of the covalent radii. He attributes these differences to partial ionic character and thus implies that partial ionic character shortens covalent bonds. This conclusion is in accord with the Schoemaker— Stevenson (1941) rule Dab = a + pb—C nx— b where > interatomic distance between A and B, rx and r = covalent radii of A and B, a and xb = electronegativity of A and B and C = constant. 

In the fourth paper of the series, Pauling (1932a) went a step further in the discussion of the partial ionic character of bonds. To the qualitative criterion for bond character presented before, he added a quantitative semiempirical criterion that enabled him to determine the approximate percentages of ionic and covalent character of bonds and then to map atoms in a scale of relative electronegativities. Pauling was thus able to suggest a viable alternative to the quantum mechanical treatment outlined in the former paper, which, as he pointed out before, was impossible to carry out except in the simplest cases. 

The Electronegativity Scale of the Elements. It has been found possible to assign to the elements numbers representing their power of attraction for the electrons in a covalent bond, by means of which the amount of partial ionic character of the bond may be estimated. This power of attraction for the electrons in a covalent bond is called the electronegativity of the element. In Figure 11-11 the elements other than the transition elements (which all have electronegativity values close to 1.6) and the rare-earth metals (which have values close to 1.3)... 

The quantitative relation between bond energy and electronegativity difference may be expressed by an equation. For a sii Ie covalent bond between two atoms A and B the extra energy due to the partial ionic character is approximately 23 (x — x ) 2 kcal/mole that is, it is proportional to the square of the difference in electronegativity of the two atoms, and the proportionality constant has the value 23 kcal/mole. For example, chlorine and fluorine have electronegativity values differing by 1 (Table 32-1) hence the heat of formation of GIF (containing... 

The experimental value is usually shortened from the covalent radius sum, and this may arise from (a) partial ionic character of the bond, or (b) the occurrence of k bonding. Unless the bond is... 

It has been pointed out4 that bonds between non-identical atoms may be considered to resonate between a covalent and an ionic structure, the bond in this way having partially covalent and partially ionic character. The resonance energy of this effect, which is usually essentially the same for a given bond in different molecules, is included in the values given for the bond energies in the nonresonating molecules discussed. 

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