Page 73 - Materials Science and Engineering An Introduction

Summary • 45

are those materials having mixed bonding: intermetallics and semi-metals. Mixed
ionic–covalent bonding for ceramics is also noted. Furthermore, the predominant
bonding type for semiconducting materials is covalent, with the possibility of an ionic
contribution.

SUMMARY
Electrons • The two atomic models are Bohr and wave mechanical. Whereas the Bohr model
in Atoms assumes electrons to be particles orbiting the nucleus in discrete paths, in wave me-
chanics we consider them to be wavelike and treat electron position in terms of a
probability distribution.
• The energies of electrons are quantized—that is, only specific values of energy are allowed.
• The four electron quantum numbers are n, l, m l , and m s . They specify, respectively,
electron orbital size, orbital shape, number of electron orbitals, and spin moment.
• According to the Pauli exclusion principle, each electron state can accommodate no
more than two electrons, which must have opposite spins.
The Periodic Table • Elements in each of the columns (or groups) of the periodic table have distinctive
electron configurations. For example:
Group 0 elements (the inert gases) have filled electron shells.
Group IA elements (the alkali metals) have one electron greater than a filled
electron shell.

Bonding Forces • Bonding force and bonding energy are related to one another according to Equations
and Energies 2.5a and 2.5b.
• Attractive, repulsive, and net energies for two atoms or ions depend on interatomic
separation per the schematic plot of Figure 2.10b.
• From a plot of interatomic separation versus force for two atoms/ions, the equilibrium
separation corresponds to the value at zero force.
• From a plot of interatomic separation versus potential energy for two atoms/ions, the
bonding energy corresponds to the energy value at the minimum of the curve.

Primary Interatomic • For ionic bonds, electrically charged ions are formed by the transference of valence
Bonds electrons from one atom type to another.
• The attractive force between two isolated ions that have opposite charges may be
computed using Equation 2.13.
• There is a sharing of valence electrons between adjacent atoms when bonding is
covalent.
• Electron orbitals for some covalent bonds may overlap or hybridize. Hybridization of
2
3
s and p orbitals to form sp and sp orbitals in carbon was discussed. Configurations
of these hybrid orbitals were also noted.
• With metallic bonding, the valence electrons form a “sea of electrons” that is uni-
formly dispersed around the metal ion cores and acts as a form of glue for them.

Secondary Bonding • Relatively weak van der Waals bonds result from attractive forces between electric
or van der Waals dipoles, which may be induced or permanent.
Bonding • For hydrogen bonding, highly polar molecules form when hydrogen covalently bonds
to a nonmetallic element such as fluorine.