Chapter 12 Notes
Modern Materials
12.1 Classes of Materials
Materials can be classified in different ways:


Types of bonds that hold the substance together – covalent-network, ionic, metallic solids
Electrical Conductivity – Insulators, semiconductors, or conductors
In order for a substance to conduct electricity the particles must be able to move and carry a charge.
Ionic solids have localized valence electrons, which leads to insulating behavior. In metallic solids, the
valence electrons are delocalized and shared collectively. Since the electrons in a metallic solid are
delocalized, it allows electrons to move freely, which results in metallic solids having high thermal and
electrical conductivity. In a covalent network, the valence electrons are localized in covalent bonds, but
in certain materials, light, heat, or an electrical field can excite the electrons and allow them to be
conducted. Such materials are called semiconductors.
12.2 Electronic Structure of Materials
Recall Molecular Orbital Theory – Atomic orbitals add to make molecular orbitals
As the number of atoms bonded together within a molecule increases, the distance between the filled,
bonding orbitals and unfilled, antibonding orbitals decreases. This energy difference between bonding
and antibonding orbitals is called the band gap.
 Note that the band gap in a metal is virtually
nonexistent, which results in high conduction.
The band gap in a semiconductor is significant,
but the gap can be influenced by a small bit of
energy.
The band gap in an insulator is too high to be
consistently overcome, which is what causes
insultation.
12.3 Semiconductors
Elemental Semiconductors: One type of atom
Ex. Silicon, germanium, gray tin, carbon as graphite (all but graphite adopt diamond’s crystal
structure)
Compound semiconductors: two or more elements
Valence band – bonding molecular orbitals
Conduction band – antibonding molecular orbitals
Compound semiconductors maintain average of 4 valence electrons per atom
Ex. GaAs (3 and 5), CdTe (2 and 6), and InP (3 and 5)
Crystal structure of compound semiconductors is often face-centered, cubic structure, where each atom
is surrounded by a tetrahedron of the opposite type of atom.
Size of the band gap within a semiconductor depends on the position of the elements on the periodic
table. Smaller elements will have increased orbital overlap, which leads to a larger splitting of the
bonding and antibonding orbitals. Also, the larger the difference in electronegsativity between the
atoms, the larger the band gap.
Semiconductor Doping
Doping with p-type semiconductors provide holes
for electrons to move through
Doping with n-type semiconductors provide extra
electrons to hop into the p-type holes
Alternating P-type and n-type semiconductors
forms the basis of diodes, transistors, solar cells,
and many other electronic devices
12.5 Superconductors
Even metals are not infinitely
conductive; there is some
resistance to electron flow due to
the vibrations of atoms and the
presence of impurities and defects.
In 1911, Dutch physicist H.
Kamerlingh Onnes discovered that
when mercury is cooled below 4.2
K, it loses all resistance to the flow
of electrical current. This is known
as superconductivity. Substances
appear to have a superconducting
transition temperature, below
which the substance is a
superconductor.
12.6 Polymers and Plastic
Polymer – molecular substances of high molecular mass formed by the polymerization of monomers,
molecules with low molecular mass.
Plastics – materials that can be formed into various shapes, usually by the application of heat and
pressure
Thermoplastic – can be reshaped
Thermoset plastic – cannot be reshaped easily
Elastomer – a material that exhibits rubbery or elastic behavior
Polymers are made by addition reactions (coupling monomers through their double bonds),
condensation polymerization (removal of water), and other ways.
Vulcanization of rubber: (a) is natural rubber, (b) is vulcanized rubber, where sulfur has been added to
cross-link the polymer chains.