Crystal Binding (Bonding) Part IIIOverview & Survey of Bonding Types Continued

Classification of Solids by Atom Type & Type of Bonding

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Classification of Solids by Atom Type & Type of Bonding

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

“Low Temperature” Solids

Insulators, Ceramics, Semiconductors

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

Summary:Bonding Characteristics & Resulting Physical Properties

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.

Chemist’s Classification of Crystals: 4 Types

Some Molecular Solids:• Many organic solids

+

Graphite • The bonding between

the planes is due to Van der Waals forces.

2. Covalent Crystals• 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 Phasefor the compounds!

Example: Diamond StructureAtoms are arranged in 4-fold coordinated

FCC unit cell, 2 atom basis

Ball-and-Stick ModelPolyhedral Modelblue C blue C

onlyonly

Hard-Sphere Model

Two of the Solid Phases of Carbon

• The Diamond phase is acovalent-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 solid phases of Carbon are interesting (to me). 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.

Two of the Solid Phases of Carbon

• 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.

• The solid phases of Carbon are interesting (to me). 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.

3. Metallic Crystals• 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.

• 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• 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. Most of these crystals are not very useful to physicists. But, some Geoscience friends are experts on these kinds of crystals.

4. Ionic Crystals• 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.

• In ionic crystals, the ions are in close packed arrangements to maximize the attractions &to minimize the repulsionsbetween 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.

Example #1: Graphite: • As we’ve already discussed, this contains covalently

bonded sheets of C with sheets loosely bound to each other by Van der Waals bonds.

Mixed Ionic & Covalent Bonding

Example #2: Mica: • Contains strongly bonded silica (SiO2)tetrahedra

sheets (mixed covalent & ionic) bound by weak ionic and hydrogen bonds

• The bonds in naturally occurring minerals have mixtures of ionic & covalent character. Many also contain more than one bond type.

Mixed Ionic & Covalent Bonding• The bonds in naturally occurring minerals have

mixtures of ionic & covalent character. Many also contain more than one bond type.

Example #3: 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