Polar Covalent Bonds, Electronegativity, and Bond Dipoles

5 Polar Covalent Bonds, Electronegativity, and Bond Dipoles 

Electrons in covalent bonds are not necessarily shared equally by the two atoms that they connect. If one atom has a greater tendency to attract electrons toward itself than the other, the electron distribution is polarized, and the bond is described as polar covalent. The tendency of an atom to attract the electrons in a covalent bond toward itself defines its electronegativity. An electronegative element attracts electrons an electropositive one donates them. 

Hydrogen fluoride, for example, has a polar covalent bond. Fluorine is more electronegative than hydrogen and pulls the electrons in the H—F bond toward itself, giving 

The covalent bond in H2 joins two hydrogen atoms. Because the bonded atoms are identical, so are their electronegativities. There is no polarization of the electron distribution, the H—H bond is nonpolar, and a neutral yellow-green color dominates the electrostatic potential map. Likewise, the F—F bond in F2 is nonpolar and its electrostatic potential map resembles that of H2. The covalent bond in HF, on the other hand, unites two atoms of different electronegativity, and the electron distribution is very polarized. Blue is the dominant color near the positively polarized hydrogen, and red the dominant color near the negatively polarized fluorine. 

The most commonly used electronegativity scale was devised by Linus Pauling. Table 1.3 keys Pauling s electronegativity values to the periodic table. 

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Chlorine is much more electronegative than carbon, so a strong electric dipole exists between each chlorine and the carbon atom. The chlorine atoms are symmetrically arranged around the carbon so that the molecule itself is not polar, even though it has four polar covalent bonds between its atoms. 

Because chlorine is more electronegative than carbon, carbon tetrachloride has four polar covalent bonds. But, as pointed out earlier, the molecular symmetry cancels out the electric dipoles of the individual bonds. The result is a nonpolar molecule. Like water, carbon tetrachloride is a good solvent. At one time, it was used as a dry cleaning agent. Water and carbon tetrachloride, however, dissolve entirely different classes of compounds. Carbon tetrachloride forms solutions with nonpolar organic compounds. It is infinitely miscible, for example, with benzene, whereas water and benzene do not mix. 

Before looking at the forces between molecules, it s first necessary to develop the ideas of bond dipoles and dipole moments. We saw in Section 7.4 that polar covalent bonds form between atoms of different electronegativity. Chlorine is more electronegative than carbon, for example, and the chlorine atom in chloromethane (CH3C1) thus attracts the electrons in the C C1 bond toward itself. The C-Cl bond is therefore polarized so that the chlorine atom is slightly electron-rich (8—) and the carbon atom is slightly electron-poor (<5+). 

Organic molecules often have polar covalent bonds as a result of unsym-metrical electron sharing caused by difterences in the electronegativity of atoms. For example, a carbon-chlorine bond is polar because chlorine attracts the shared electrons more strongly than carbon does. Carbon-hydrogen bonds are relatively nonpolar. Many molecules as a whole are also polar owing to the cumulative effects of individual polar bonds and electron lone pairs. The polarity of a molecule is measured by its dipole moment, p. 

Polar Covalent Bonding Electronegativity and Dipole Moments... 

The separation of charge in a polar covalent bond creates an electric dipole. We expect the dipoles in the covalent molecules HF, HCl, HBr, and HI to be different because F, Cl, Br, and I have different electronegativities. This tells us that atoms of these elements have different tendencies to attract an electron pair that they share with hydrogen. We indicate this difference as shown here, where A(EN) is the difference in electronegativity between two atoms that are bonded together. 

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. ... 

If the difference in electronegativity (ts.EN) between two atoms is between 0.4 and 1.7, we expect the bond between them to be a polar covalent bond. For example, hydrogen has an electronegativity of 2.20, and chlorine has an electronegativity of 3.16. The difference in electronegativity (AEN) is 0.96, so the bond is a polar covalent bond, which is sometimes called a bond dipole. 

Polar covalent bond (Section 1.3) Any shared electron bond between two different atoms must be polar. Two nonidentical atoms must have different electronegativities and will attract the shared electrons to different extents, creating a dipole. 

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