Page 48 - Materials Science and Engineering An Introduction

WHY STUDY Atomic Structure and Interatomic Bonding?

An important reason to have an understanding of addition, the electrical properties of diamond and
interatomic bonding in solids is that in some instances, graphite are dissimilar: diamond is a poor conductor of
the type of bond allows us to explain a material’s electricity, but graphite is a reasonably good conductor.
properties. For example, consider carbon, which may These disparities in properties are directly attributable
exist as both graphite and diamond. Whereas graphite to a type of interatomic bonding found in graphite that
is relatively soft and has a “greasy” feel to it, diamond does not exist in diamond (see Section 12.4).
is one of the hardest known materials in nature. In

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Name the two atomic models cited, and note (b) Note on this plot the equilibrium separation
the differences between them. and the bonding energy.
2. Describe the important quantum-mechanical prin- 4. (a) Briefly describe ionic, covalent, metallic, hy-
ciple that relates to electron energies. drogen, and van der Waals bonds.
3. (a) Schematically plot attractive, repulsive, and (b) Note which materials exhibit each of these
net energies versus interatomic separation bonding types.
for two atoms or ions.

2.1 INTRODUCTION
Some of the important properties of solid materials depend on geometric atomic ar-
rangements and also the interactions that exist among constituent atoms or molecules.
This chapter, by way of preparation for subsequent discussions, considers several fun-
damental and important concepts—namely, atomic structure, electron configurations in
atoms and the periodic table, and the various types of primary and secondary intera-
tomic bonds that hold together the atoms that compose a solid. These topics are re-
viewed briefly, under the assumption that some of the material is familiar to the reader.

Atomic Structure

2.2 FUNDAMENTAL CONCEPTS
Each atom consists of a very small nucleus composed of protons and neutrons and is
encircled by moving electrons. Both electrons and protons are electrically charged, the
1
charge magnitude being 1.602 10 19 C, which is negative in sign for electrons and positive
for protons; neutrons are electrically neutral. Masses for these subatomic particles are
extremely small; protons and neutrons have approximately the same mass, 1.67 10 27 kg,
which is significantly larger than that of an electron, 9.11 10 31 kg.
Each chemical element is characterized by the number of protons in the nucleus,
atomic number (Z) or the atomic number (Z). 2 For an electrically neutral or complete atom, the atomic
number also equals the number of electrons. This atomic number ranges in integral units
from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring elements.
The atomic mass (A) of a specific atom may be expressed as the sum of the masses of
protons and neutrons within the nucleus. Although the number of protons is the same for

1 Protons, neutrons, and electrons are composed of other subatomic particles such as quarks, neutrinos, and bosons.
However, this discussion is concerned only with protons, neutrons, and electrons.
2 Terms appearing in boldface type are defined in the Glossary, which follows Appendix E.
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