Physical Chemistry Third Edition

878 21 The Electronic Structure of Polyatomic Molecules

EXAMPLE21.3

Are the bonds in the CH 4 molecule polar or nonpolar? Does the CH 4 molecule have a nonzero

dipole moment?

Solution

The four single bonds are slightly polar, since the electronegativity of carbon on the Pauling

scale is 2.5 and that of hydrogen is 2.1. The molecule is tetrahedral, with each C–H bond

formed from a 2sp^3 hybrid orbital on the carbon atom, and with the bonds at the tetrahedral

angle (109.5◦) from each other. The dipole contributions from the four bonds cancel each

other so that the molecule is nonpolar.

Exercise 21.5

Describe the bonding in ammonia, NH 3 , including the conformation of the molecule and a

statement about the dipole moment of the molecule. How do you think the actual bond angles

deviate from the tetrahedral angle of 109.5◦?

PROBLEMS

Section 21.3: The CH 4 ,NH 3 , and H 2 O Molecules and the

sp^3 Hybrid Orbitals

21.11Show that the 2sp^3 hybrid orbitals of Eq. (21.10-1) are
normalized and are orthogonal to each other.

21.12Describe the bonding in the HF molecule usingsp^3
hybrids for the fluorine instead ofsphybrids. Which
description fits better with the VSEPR theory?

21.13Describe the bonding in the F 2 molecule usingsp^3
hybrids instead of the orbitals used in Table 20.2.

Which description fits better with the VSEPR

theory?

21.14Using LCAOMOs made with hybrid orbitals, describe the

bonding and molecular shape of each of the molecules or

ions:

a.SO^24 −

b.BF 3

c.HClO 4

21.4 Molecules with Multiple Bonds

According to the description in Chapter 20, the double bond in C 2 consists of two pi

bonds. We now construct approximate wave functions for larger molecules containing

double bonds.

EXAMPLE21.4

Describe the bonding in ethene (ethylene), which has the following structural formula

H

C

HH

H

C