Chemistry, Third edition

70 5 · MORE ABOUT BONDING

BOX 5.1

Finding out molecular shapes

How do we know that VSEPR theory is good

for predicting the shapes of molecules?

The predictions have been shown to be true

by a number of techniques which can be

used to gain information about the shapes

of molecules:

1. X-ray diffractionX-ray diffraction can
   be used to determine the arrangement
   of atoms in a crystal. Simply put, this
   involves using X-rays (which are
   electromagnetic radiation with
   wavelengths about the same as the
   distances between neighbouring atoms
   in a crystal) to take ‘pictures’ of the
   crystal structure.


2. Infrared spectroscopyA pair of
   covalently bonded atoms vibrate when

they absorb electromagnetic radiation.

The frequency of this radiation depends

on the atoms involved and upon the

strength of the bond between them. The

infrared spectrum of a molecule

measures its absorbance over the

infrared region of the electromagnetic

spectrum and the spectrum depends, in

part, on the geometry of the molecule.

3. Dipole moment measurementsWe
   have already discussed polar covalent
   bonds. Molecules which contain these
   bondsmayhave a positive end and a
   negative end (a dipole), depending on
   their geometry. Measurement of the
   magnitude of dipoles (the dipole
   moment), therefore, could give valuable
   information about the shape of a

molecule. Dipole moments are discussed

in the next section.

4. Microwave spectroscopyMolecules
   rotate in the gaseous state. Upon absorb-
   ing microwave radiation, the molecules
   rotate faster. The frequencies of radiation
   absorbed by molecules and the relative
   intensities of such absorptions depend
   upon the geometry of the molecules. A
   mathematical analysis of the microwave
   spectrum of a molecule can provide
   chemists with bond angles and bond
   lengths. This information is much more
   accurate than that obtained by infrared
   spectroscopy. Study of the effect of a
   magnetic field on a microwave spectrum
   allows the dipole moment of the molecule
   to be calculated.

Example 5.5 (continued)

therefore a distorted trigonal planar shape with respect to the electron pairs, or

V-shaped with respect to the S=O bonds:

Comment

The angle between the S=O bond, at 119.5°, is less than the usual trigonal planar

angle of 120°. The S=O bonds are ‘pushed’ together by the repulsion of the lone

pair, but the double bonds have greater electron density than single bonds so the

effect is not so pronounced.

Shapes of molecules with multiple bonds

What are the shapes of the following?

Exercise 5F

(i) the carbonate ion, CO 32 

(ii)sulfur trioxide, SO 3

(iii)carbon disulfide, CS 2 (draw the Lewis

structure, then the structural formula

first!)

(iv)the sulfate ion, SO 42 

(v)the nitrate ion, NO 3 

O—C—O



O

OSO



O

2–

O—N—O



O

O



O—S—O



O

2–