Bonds covalent, with partial ionic character

Instead of using this description of the bond as involving resonance between an extreme covalent bond H C1 and an extreme ionic bond H+Cl-, we may describe the bond as a covalent bond with partial ionic character, and make use of the valence line, writing H—Cl (or H—Cl )... 

Often only the covalent structure is shown, and the chemist bears in mind that the covalent bonds have a certain amount of ionic char acter. These bonds are called covalent bonds with partial ionic character. 

The coefficients of ij/i and i/ n the composite description are equal, indicating that these two contribute equally to the structure. The coefficient of differs from the other two, indicating that i/ ni contributes differently. The contributions of the three structures in HCl are estimated to have the values I, 26 % II, 26 % III, 48 %. The structures I and II are covalent structures, so we may say that the bond in HCl is 52 % covalent and 48 % ionic. A bond in which the ionic contribution is significant is called a covalent bond with partial ionic character. 

Within the molecule, this situation is usually described in terms of the covalent bond having a partial ionic character, and this will affect its properties and behavior. Between two (or more) molecules with dipole moments, there will be additional interactions arising from these electrostatic dipoles. 

These argumenls can also be presented in terms of covalent bonds with partial ionic character. 

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. 

Because the Cl atom is more electronegative than the H atom, the bond pair is pulled towards the Cl atom. This results in a covalent bond with some partial ionic character the small charges present are indicated by a 8. 

These covalent radii are for use in molecules in which the atoms form covalent bonds to a number determined by their positions in the periodic table—carbon four, nitrogen three, and so on. It is found empirically that the radii are applicable to covalent bonds with considerable ionic character for extreme ionic bonds, however, ionic radii are to be used (Chap. 13), and in some molecules, discussed in later sections, the partial ionic character plays an important part in determining the interatomic distances. 

Fig. 2). Electrical dipoles are thus created, the overall effect of which places a fractional negative charge (—5) on the chlorine atom and a fractional positive charge (+5) on the carbon atom. By analogy with the extreme case in which chlorine captures both shared electrons and forms an electrovalent ion pair [e.g., Na Cl — (Na) + ( Cl) ], such a covalent bond is said to have partial ionic character. ... 

If for any given compound two or more structures can be written differing only in the distribution of electronSj the properties of that compound will not be those to be expected of any of the formulas but rather they will be those to be expected of a hybrid of them all. An ion or a molecule in which resonance can occur will always be more stable than one-in which it cannot. It is obvious that when the principle is stated in such general terms no attempt has been made to explain the phenomenon, but rather the conditions have been described for recognizing its occurrence. Furthermore, since each covalent bond may have "a certain amount of partial ionic character, different electronic structures may actually be written for every chemical compound. The problem with which we are con-... 

Polar covalent bonds may be thought of as intermediate between pure (nonpolar) covalent bonds and pure ionic bonds. In fact, bond polarity is sometimes described in terms of partial ionic character. This usually increases with increasing difference in electronegativity between bonded atoms. Calculations based on the measured dipole moment of gaseous HCl indicate about 17% ionic character. ... 

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