Overview & Survey of Bonding Types Continued

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
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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

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 +

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 Phase for the compounds!

Example: Diamond Structure
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 covalentnetwork solid in which C
’s are strongly covalently bonded ( sp 3 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 ( sp 2 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
• 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 .
• 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
( SiO
2
)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