Chapter 12 – Solids and Modern Materials
11.1 – Classification of Solids
• Solids are classified by the types of bonds that hold
the atoms in place.
• Metallic solids are held together by a “sea” of
collectively shared electrons.
• Ionic solids are sets of cations and anions mutually
attracted to one another.
• Covalent-network solids are joined by an extensive
network of covalent bonds.
• Molecular solids are molecules held together by
intermolecular bonds. These forces are weak, so
molecular solids usually have low melting points.
Other types of solids
• Polymers contain long chains of atoms (usually
carbon) connected by covalent bonds; the chains can
be connected to other chains by weak intermolecular
forces. They usually have higher melting points than
molecular solids, and are generally flexible.
•
polypropylene
• Nanomaterials are crystalline compounds with the
crystals on the order of 1–100 nm; this gives them
very different properties than larger crystalline
materials.
12.2 – Structures of Solids
• Solids with a regular repeating pattern of atoms are
crystalline.
• Amorphous solids are characterized by a distinct lack
of order in the arrangement of atoms.
• The repeating unit of a crystalline solid is the unit
cell. The unit cells are arranged to form the crystal
lattice.
•
Crystal Lattice
12.3 – Metallic Solids
• Alloys are materials that contain more than one
element and have the characteristic properties of
metals. Alloys are made so as to modify the
properties of pure metals. Ex. bronze, brass,
stainless steel, pewter.
Types of Alloys
• Substitutional alloys: A second element takes the place of a
metal atom. They form when the 2 metals are similar in size.
• Interstitial alloys: A second element fills the “holes” between
metal atoms. The solute element is much smaller than the
solvent element.
• Heterogeneous alloys: components not dispersed uniformly
The heterogeneous alloy
pearlite
12.4 – Metallic Bonding
• Electron-sea model: a
metal is a group of cations
suspended in a sea of
electrons.
• The electrical and thermal
conductivity, ductility, and
malleability of metals is
explained by this model.
• Since the electrons are
mobile, and aren’t confined
to any particular ion, they
will flow through a wire
towards the + end when
voltage is applied
• Metals conduct heat because the movement of electrons in
response to temperature differences allows for the transfer of
kinetic energy through the metal.
• Metals are malleable and ductile because the mobile
electrons allow for the metal atoms to reposition themselves
12.5 – Ionic Solids
• In ionic solids, the lattice comprises
alternately charged ions.
• Because ionic bonds would have to be
broken to melt or boil an ionic
compound, ionic solids have very high
melting and boiling points.
• They are brittle because of the repulsive
interactions between ions of like charge
when a stress applied to the solid shifts
the ions.
• Ionic solids are usually electrical
insulators, because the valence electrons
are confined to the anions, not mobile.
• Melting and boiling points of ionic solids depends on the
charges and sizes of the ions.
• The smaller the ions, the higher the m.p. and b.p.
• The greater the charges, the higher the m.p. and b.p.
• For example, the m.p. of NaCl is 801°C and the m.p. of MgO is
2852°C
12.6 – Molecular Solids
• Because of the weak intermolecuar forces (dispersion, dipoledipole, and H-bonds) that hold them together, they are soft
and have relatively low melting points (usually below 200°C).
• Ex. Ice (m.p. 0°C), sucrose (m.p. 184°C)
Sucrose
12.7 – Covalent-Network Solids
• They have much higher melting points and are much harder
than molecular solids, since covalent bonds are much stronger
than intermolecular forces
• Ex. diamonds, graphite, quartz, silicon carbide
quartz
Silicon carbide
• In a diamond, each carbon is bonded to 4 other carbons
(tetrahedral, sp3). It’s the hardest known material.
• In graphite, each carbon is bonded to three other carbons
(trigonal planar, sp2) and has delocalized electrons (π bonds).
The layers are held together by dispersion forces.
Semiconductors
• Some metalloids, such as silicon and germanium, are
semiconductors, meaning they conduct electricity somewhat,
but not as well as metals. Their electrical conductivity is
increased by the presence of impurity atoms, added in a
process called doping.