Crystal Binding (Bonding) Overview & Survey of Bonding Types Continued

Classification of Solids by Atom Type & Type of Bonding This chart continues on the next page from where it stops at the bottom of this one!

Classification of Solids by Atom Type & Type of Bonding (Continued) The chart on the previous page continues from the bottom with:

Classification of Solids by Atom Type & Type of Bonding (Continued)

Classification of Solids by Atom Type & Type of Bonding (Continued) “Low Temperature” Solids Insulators, Ceramics, Semiconductors

Summary: Bonding Characteristics & Resulting Physical Properties

Chemist’s Classification of Crystals: 4 Types 1. Molecular Crystals These consist of neutral molecules held together by weak Van der Waals bonds. The weak bonds mean that the solid can relatively easily decompose, melt, etc. Some Molecular Solids: Many organic solids Graphite  The bonding between the planes is due to Van der Waals forces.

2. Covalent Crystals Zincblende Phase These consist of atoms of similarly high electro-negativity either from right side of the Periodic Table or from the middle of the Periodic Table (Columns II, III, IV, V, IV.) Networks of strong covalent bonds with no weak links. Directional bonds with low symmetry and density. Covalent Solids: C in the Diamond Phase  Si, Ge, α-Sn, GaAs, InAs, ... Zincblende Phase for the compounds!

Example: Diamond Structure FCC unit cell, 2 atom basis Atoms are arranged in 4-fold coordinated FCC unit cell, 2 atom basis Hard-Sphere Model Ball-and-Stick Model blue C only Polyhedral Model

Two of the Solid Phases of Carbon The solid phases of Carbon are interesting. The two most common solid phases are Diamond & Graphite. These solids contain the same kind of atoms. However, they have very different bonding types, which leads to very different properties. The Diamond phase is a covalent-network solid in which C’s are strongly covalently bonded (sp3 bonds) to each other. It is the hardest material known & it has a high melting point. The Graphite phase: Within each plane, the C’s are strongly covalently bonded (sp2 bonds) in a 2 D covalent network. The planes are weakly bonded to each other with Van der Waals forces as in a molecular solid. Graphite is very soft & has a low melting point.

3. Metallic Crystals  Metals! Metallic Bonds These consist of atoms of similar electro-negativity & usually from toward left side of the Periodic Table.  Metals! Metallic Bonds 1. Are directionless, with high symmetry & density. 2. Are mostly between atoms in a Close Packed arrangement.  With 12 nearest neighbors. If the crystal is cubic, it will be Cubic Close Packed (abcabcabc stacking) so that the structure is FCC. Some metals are hexagonal & are Hexagonal Close Packed (ababab stacking) so that the structure is HCP. Some metals have the BCC structure, which is not a Close Packed arrangement.

The valence electrons are delocalized Metallic crystals are not covalently bonded, but the attractions between atoms are too strong to be Van der Waals forces. As we’ve said, in metals The valence electrons are delocalized throughout the solid.

4. Ionic Crystals Large Electronegativity Differences. As we’ve already said, these consist of atoms with Large Electronegativity Differences. Most naturally occurring minerals are ionic crystals. Further, many of these minerals are oxides. As a first approximation, these oxides can be thought of as an array of oxygen atoms in a close packed arrangement, with metallic ions fitting into interstitial sites between the oxygens. Most of these crystals are not very useful to phyicists. But, some Geoscience friends are experts on these kinds of crystals. In ionic crystals, the ions are in close packed arrangements to maximize the attractions & to minimize the repulsions between the ions.

Mixed Ionic & Covalent Bonding The bonds in naturally occurring minerals have mixtures of ionic & covalent character. Many also contain more than one bond type. Examples Graphite: Contains covalently bonded sheets of C with sheets loosely bound to each other by Van der Waals bonds. Mica: Contains strongly bonded silica (SiO2)tetrahedra sheets (mixed covalent & ionic) bound by weak ionic and hydrogen bonds Fluorite, Halite, Calcite: Contain cleavage planes commonly bound to planes of weak ionic bonding in an otherwise tightly bound structure.

Summary of Chemists Classification of Solids: Bonding Characteristics & Resulting Properties