Absolutely Small-How Quantum Theory Explains Our Everyday World by Michael D. Fayer

Author:Michael D. Fayer
Language: eng
Format: mobi
Tags: Science, Non-Fiction
ISBN: 9780814414880
Publisher: AMACOM/American Management Association
Published: 2010-06-16T00:00:00+00:00

12

The Hydrogen Molecule and the Covalent Bond

ONE OF THE GREAT TRIUMPHS of quantum mechanics is the theoretical explanation of the covalent bond. Two types of interactions hold atoms together, covalent bonds and ionic bonds. Ionic bonds are the type that occurs in a sodium chloride (NaCl) crystal. We know from Chapter 11 and our discussion of the Periodic Table that this salt crystal is composed of sodium cations, Na+1, and chloride anions, Cl-1. The ions in the crystal are held together by electrostatic interactions. Opposite charges attract. There are some complications because like charges repel, but it is possible to show that the attractions of the oppositely charged ions overcome the repulsions of the like charged ions. Such electrostatic interactions can be explained quite well with classical mechanics, although quantum theory is still needed to explain many properties in detail.

In contrast to ionic solids that are held together by electrostatic interactions, classical mechanics cannot explain the covalent bond. We saw in Chapter 11 that a hydrogen atom will tend to form one covalent bond with another atom to share one electron. This sharing brings the H atom to the helium closed shell configuration. But what is a covalent bond? Why do H atoms share electrons to form the H2 molecule, but helium atoms do not share electrons to form the He2 molecule? We will first investigate the nature of the covalent bond for the simplest molecule, H2, and then expand the discussion of covalent bonding for more complicated molecules in subsequent chapters. By the end of this chapter it will be clear why H2 exists and He2 does not exist.

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