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S SCIENCE HAS MATURED, it has developed the perspective that the

physical world is regular and that its behavior follows certain rules and

guidelines. By the 1800s, chief among these rules were the laws of mechanics

that explained the motion of bodies of matter; specifically, Newton’s three laws

of motion. Scientists felt confident that they were beginning to understand the

natural world and how it worked.

Early in the 1800s, and certainly by the middle and end of the century, how-

ever, little hints began to appear suggesting that scientists really didn’t under-

stand what was going on. Or, rather, that the accepted physical laws neither ap-

plied to nor predicted certain events. Toward the end of the nineteenth century,

it was obvious to a few radical thinkers that a new theory describing the be-

havior of matter would be necessary in order to understand the nature of the

universe. Finally, in 1925–1926, a new theory named quantum mechanics was

shown to accurately account for the new observations that did not fit with the

earlier, classical mechanics.

In order to fully appreciate quantum mechanics and what it provides for

chemists, it is crucial to review the state of physical science immediately before

quantum mechanics. In this chapter, we review classical mechanics and discuss

the phenomena that classical mechanics did not explain. Although it may not

seem like chemistry at first, remember that a major goal in physical chemistry

is to model the behavior of atoms and molecules. Since the chemically most im-

portant parts of the atom are the electrons, a proper understanding of electron

behavior is absolutely necessary to any understanding of chemistry. Because

the electron had been shown to be a piece of matter, classical scientists tried to

use classical equations of motion to understand the behavior of the electron.

However, they soon discovered that the old models didn’t work for such a small

piece of matter. A new model had to be developed, and quantum mechanics

was that model.

9.1 Synopsis

In this chapter, we start with a review of how scientists classify the behavior of

the motion of matter. There are several mathematical ways to describe motion,

Newton’s laws being the most common. A quick historical review shows that

9.1 Synopsis

9.2 Laws of Motion

9.3 Unexplainable Phenomena

9.4 Atomic Spectra

9.5 Atomic Structure

9.6 The Photoelectric Effect

9.7 The Nature of Light

9.8 Quantum Theory

9.9 Bohr’s Theory of the

Hydrogen Atom

9.10 The de Broglie Equation

9.11 Summary: The End of
Classical Mechanics

Pre-Quantum Mechanics