1.1.1 Effects of Structure on Properties
Physical properties of metals, ceramics, and polymers, such as ductility, thermal expansion, heat
capacity, elastic modulus, electrical conductivity, and dielectric and magnetic properties, are a direct
result of the structure and bonding of the atoms and ions in the material. An understanding of the
origin of the differences in these properties is of great engineering importance.
In single crystals, a physical property such as thermal expansion varies with direction, reflecting
the crystal structure; whereas in polycrystalline and amorphous materials, a property does not vary
with direction, reflecting the average property of the individual crystals or the randomness of the
amorphous structure. Most engineering materials are polycrystalline, composed of many grains, and
thus an understanding of the properties requires not only a knowledge of the structure of the single
grains but also a knowledge of grain size and orientation, grain boundaries, and other phases present;
that is, a knowledge of the microstructure of this material.
1.1.2 Atomic Structure
Atoms consist of electrons, protons, and neutrons. The central nucleus consists of positively charged
protons and electrically neutral neutrons. Negatively charged electrons are in orbits about the nucleus
in different energy levels, occupying a much larger volume than the nucleus.
In an atom, the number of electrons equals the number of protons and, hence, an atom is neutral.
The atomic number of an element is given by the number of protons, and the atomic weight is given
by the total number of protons and neutrons. (The weight of the electrons is negligible.) Thus,
hydrogen, H, with one proton and one electron, has an atomic number of 1 and an atomic weight of
1 and is the first element in the periodic chart. Oxygen, O, with atomic number 8, has eight protons
and eight neutrons and, hence, an atomic weight of 16.
Completed electronic shells have a lower energy than p