NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 52

2

NATURE OF MATERIALS

To a large extent, the behavior of materials is dictated by the structure and
bonding of the atoms that are the building blocks for all matter. Knowledge
of the bonding and structure of materials at the molecular level allows us to
understand their behavior. The chapter presents a broad overview of con-
cepts essential to our understanding of these behaviors. The chapter reviews
the basic types of bonds and then, based on the type of bond, classifies mate-
rials as metallic, ceramic, or amorphous. The general nature of each of these
classes of materials is presented.

2.1

Basic Materials Concepts

Atoms are the basic building blocks of all materials. For the purpose of this
text, atoms will be considered to consist of three subatomic particles:
protons, neutrons, and electrons. The protons and neutrons are at the center
of the atom, while the electrons travel about the nucleus in paths or shells.
The atomic number is the number of protons in the nucleus of the atom. The
atomic mass of an atom is the number of protons plus the number of neu-
trons in the center of the atom. An element is an atom or group of atoms with
the same atomic number. Isotopes are elements with different numbers of
neutrons in the nucleus.

■ 2.1.1 Electron Configuration

The behavior of an atom's electrons controls, to a large extent, the character-
istics of an element. An electrically neutral (or complete) atom has equal num-
bers of electrons and protons. However, an atom may either release or attract
electrons to reach a more stable configuration. Electrons travel around the
nucleus in orbital paths, or orbits, around the nucleus. The distance between
electrons and the nucleus is not fixed; it is better described as a random vari-
able with a distribution, as shown in Figure 2.1.