Page 62 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 4

Electronic

Configuration

of the Atom

4.1. INTRODUCTION
In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons
are present in the atom and that electrons are arranged in shells allow us to explain isotopes (Chap. 3), chemical
bonding (Chap. 5), and much more. However, with this simple theory, we still have not been able to deduce why
the transition metal groups and inner transition metal groups arise, and many other important generalities. In this
chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer
some of these more difficult questions.
The modern theory of the electronic structure of the atom is based on experimental observations of the
interaction of electricity with matter, studies of electron beams (cathode rays), studies of radioactivity, studies of
the distribution of the energy emitted by hot solids, and studies of the wavelengths of light emitted by incandescent
gases. A complete discussion of the experimental evidence for the modern theory of atomic structure is beyond
the scope of this book. In this chapter only the results of the theoretical treatment will be described. These results
will have to be memorized as “rules of the game,” but they will be used so extensively throughout the general
chemistry course that the notation used will soon become familiar. In the rest of this course, the elementary
theory presented in Chap. 3 will suffice. You should study only those parts of this chapter that are covered in
your course.

4.2. BOHR THEORY
The first plausible theory of the electronic structure of the atom was proposed in 1914 by Niels Bohr (1885–
1962), a Danish physicist. To explain the hydrogen spectrum (Fig. 4-1), he suggested that in each hydrogen
atom, the electron revolves about the nucleus in one of several possible circular orbits, each having a definite
radius corresponding to a definite energy for the electron. An electron in the orbit closest to the nucleus has the
lowest energy. With the electron in that orbit, the atom is said to be in its lowest energy state, or ground state.
If a discrete quantity of additional energy were absorbed by the atom in some manner, the electron might be
able to move into another orbit having a higher energy. The hydrogen atom would then be in an excited state.
An atom in the excited state will return to the ground state and give off its excess energy as light in the process.
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