Home > Molecular Geometry and Bonding Theories > Introduction >
     
Molecular Geometry and Bonding Theories
Introduction

We have seen in Chapter 8 that Lewis structures help us understand the compositions of molecules and their covalent bonds. However, they do not show one of the most important aspects of molecules—their overall shapes. Molecules have shapes and sizes that are defined by the angles and distances between the nuclei of their component atoms.

The shape and size of a molecule of a particular substance, together with the strength and polarity of its bonds, largely determine the properties of that substance. Some of the most dramatic examples of the important roles of molecular shape and size are seen in biochemical reactions. For example, a small change in the shape or size of a drug molecule may enhance its effectiveness or reduce its side effects. We saw an example of the influence of molecular shape on drug action in the item “The Search for a Super-aspirin,” which was part of the “Chemistry in the News” box in Chapter 1.

The sensations of smell and vision depend in part on molecular shape. When you inhale, molecules in the air are carried past receptor sites in your nose. If the molecules have the right shape and size, they can fit properly on these receptor sites, which transmit impulses to the brain. The brain then identifies these impulses as a particular aroma, such as the aroma of freshly baked bread. The nose is so good at molecular recognition that two substances may produce different sensations of odor even when their molecules differ as subtly as your right hand differs from your left.

Our first goal in this chapter is to learn the relationship between two-dimensional Lewis structures and three-dimensional molecular shapes. Armed with this knowledge, we can then examine more closely the nature of covalent bonds. The lines that are used to depict bonds in Lewis structures provide important clues about the orbitals that molecules use in bonding. By examining these orbitals, we can gain a greater understanding of the behavior of molecules. You will find that the material in this chapter will help you in later discussions of the physical and chemical properties of substances.



Copyright © 1995-2010, Pearson Education, Inc., publishing as Pearson Prentice Hall Legal and Privacy Terms