Chapter 12 – Solids and Modern Materials11.1 – 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, dipole-

dipole, 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.