Page 62 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

J `~ Solidification Single Section 1.2 Types of Atomic Bonds

Products: solid-state

waist . , l9;1.Qf‘?l, <,, t C'”'Sta'S crystals devices, turbine blades
Products: paper clips,
Polycrystals bolts, springs, I-beams,
aircraft fuselage
Body-centered cubic Lattice
Face-centered cubic lmperfections
Grain boundaries
Hexagonal Dislocations
close-packed Plastic deformation
Anlsotropy
Allotroplsm
FIGURE l.l An outline of the topics described in Chapter 1.

Chapter 7 (on polymers), Chapter 8 (ceramics and glasses), and Chapter 9 (composite
materials). The structure of metal alloys, the control of their structure, and heat-
treatment processes are described in Chapter 4.

l.2 Types of Atomic Bonds

All matter is made up of atoms containing a nucleus of protons and neutrons and
surrounding clouds, or orbits, of electrons. ln recent decades, a large number of sub-
atomic particles have been identified, with additional complexities in the nucleus. For
the purposes of this textbook, however, the planetary model of a nucleus orbited by
electrons is sufficient.
The number of protons in the nucleus determines whether an atom will be
metallic, nonmetallic, or semimetallic. An atom with a balanced charge has the same
number of electrons as protons; when there are too many or too few electrons, the
atom is called an ion. An excess of electrons results in a negatively charged atom,
referred to as an anion, while too few electrons results in a positively charged atom,
called a cation. The number of electrons in the outermost orbit of an atom deter-
mines the chemical affinity of that atom for other atoms.
Atoms can transfer or share electrons; in doing so, multiple atoms combine to
form molecules. Molecules are held together by attractive forces called bonds
through electron interaction. The basic types of atomic attraction associated with
electron transfer, called primary or strong bonds, are as follows:

° Ionic bonds. When one or more electrons from an outer orbit are transferred
from one material to another, a strong attractive force develops between the
two ions. An example is that of sodium (Na) and chlorine (Cl) in common
table salt; such salt consists of Na+ and Cl` ions (hence the term ionic bond),
which are strongly attracted to each other. Also, the attraction is between all
adjacent ions, allowing crystalline structures to be formed, as discussed in
Section 1.3. Molecules with ionic bonds generally have poor ductility and low
thermal and electrical conductivity.
° Covalent bonds. ln a covalent bond, the electrons in outer orbits are shared by
atoms to form molecules. The number of electrons shared is reflected by terms
such as “single bond,” “double bond,” etc. Polymers consist of large molecules
that are covelantly bonded together; water (H2O) and nitrogen gas (NZ) are addi-
tional common examples of molecules formed from covalent bonds. Solids formed