To understand the properties of cell membranes, proteins, and nucleic
acids, it is necessary to have knowledge of how the molecules forming
these biological components respond to the different interactions in their
environment. At the turn of the nineteenth century much of what is
now called classical physics had been developed, allowing scientists to
understand the world around them. For example, the laws of motion intro-
duced by Isaac Newton in the seventeenth century explained the motion
of everyday objects as well as planets. However, several experimental results
that were reproducible could not be explained based upon the classical
science. This led some to re-examine the basic scientific principles and to
ask how these principles could be modified to explain the experimental
results. In a tremendous burst of intellectual effort, modification of the
existing scientific theories lead to quantum mechanics, which is discussed
in these next few chapters, as well as to relativity and gravitational theory.
The new theories not only provided answers to the previously inscrut-
able experimental results but also led to many new directions for scientific
inquiry.
In this chapter, the way in which classical physics characterized the
physical world is summarized followed by a description of how certain
experiments were in conflict with the behavior predicted using the classical
theory. Presented next is the introduction of several nonclassical con-
cepts that were used to understand these experimental results and gave
rise to the development of quantum mechanics, which is applied in the
following chapters to describe the properties of atoms and molecules.
Classical concepts
A classical particle is described by a number of parameters: mass, m,
position, r, velocity, 9 , and charge, q. For simplicity, these are regarded