Chapter 10: Solids - Definitions
Crystalline Solid: a solid material in which the atoms or molecules are packed closely together in an orderly
3-D lattice. A crystalline solid, if formed slowly, will have flat, well-defined faces with definite angles at each
edge.
Amorphous Solid: a solid material in which the atoms or molecules are packed closely together in a disorderly
fashion.
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Types of Crystalline Solids
a. Metallic Crystals (Metals) - metal cations held together by a 'sea' of valence electrons. Conduct
electricity, are mallable, and have relatively low melting points. (s and d-block elements)
b. Ionic Solids - cations and anions are held together by their opposite electrostatic charges. The ionic
solids are insulators (although the ionic melt is conductive), are brittle, and have high melting points.
(NaCl, CaCO3, NaOH)
c. Molecular Solids - molecules are held together by intermolecular forces. Since these forces are
relatively weak, molecular solids generally have low melting points. Molecular solids are insulators.
(sulfur, iodine, ice)
d. Network Solids - atoms are held together by covalent bonds. Are insulators, brittle, and have very high
melting points. (diamond, graphite, silicon)
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Alloys: a mixture of two or more metals. Examples ...
Brass - mixture of copper and zinc.
Bronze - mixture of copper with another metal, such as tin and/or lead.
Solder - mixture of tin and lead
Lead Free Solder - mixture of tin and silver
Stainless Steel - mixture of iron and chromium (some carbon is also added)
Substitutional alloy - metal atoms of one type are substituted in place of metal atoms of the other type in
the metal lattice.
Interstitial alloy - relatively small metal atoms reside in the 'interstitial holes' that exist in an otherwise
pure metal lattice.
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Cubic Close-Packed Lattices: many substances adopt a 3-D lattice geometry in which the atoms, treated as
hard-spheres, are packed together as efficiently as possible so that a certain number of the atoms are touching.
a. Primitive cubic - the simplest repeat unit of the lattice (called the unit cell) consists of a cube with
an atom occupying each of the eight corners. The atoms along each edge are touching. The
coordination number (number of atoms immediately adjacent to a given atom in a crystal) is 6.
b. Body-centered cubic (bcc) - looks like the primitive cubic lattice, but an additional atom has been
placed at the center of the cube. The central atom pushes the remaining atoms slightly apart.
Therefore, the atoms along any diagonal through the cube are the only atoms touching. The
coordination number is 8.
c. Face-centered cubic (fcc) - looks like the primitive cubic lattice, but additional atoms have been
placed in the middle of each face. The atoms along any face diagonal are the only atoms touching.
The coordination number is 12.