Module H: The Solid State

Nov 3

Review: The Solid State • Particles are held in an ordered arrangement.

• Stronger Inter-Molecular Forces than liquid or gas phases.

Solids • Solids can be classified as:

1. Amorphous – No ordered long-range structure e.g. rubber

2. Crystalline - Ordered arrangement over a long range. Can be subdivided into various groups:

• Ionic Crystals

• Molecular Crystals

• Giant Covalent Networks

• Metallic Crystals

Ionic Crystals

• Ions held together by electrostatic forces e.g. NaCl, LiCl

Molecular Crystals

• Separate molecules held together by intermolecular forces e.g. sucrose and ice

Giant Covalent Networks

• Atoms are linked together by covalent bonds into a giant three-dimensional array.

• A covalent-network solid is one very large molecule.

• E.g. Diamond or quartz

Type of Solid Intermolecular Force Properties Examples

Ionic Ion-ion forces Brittle, hard, high-melting

NaCl, KBr, MgCl2

Molecular Dispersion forces, dipole-dipole forces, hydrogen bonds

Soft, low-melting, non-conducting

H2O, Br2, CO2, CH4

Giant Covalent Network

Covalent Bonds Hard, high-melting C (diamond), SiO2

Metallic Metallic Bonds Variable hardness and melting point, conducting

Na, Zn, Cu, Fe

Metallic Crystal systems. • Particles pack together so that they can be as close as possible

and maximise their interactions.

• Seven major crystal systems:

1. Cubic

2. Hexagonal

3. Tetragonal

4. Trigonal Rhombic/Orthorhombic

5. Rhombic

6. Monoclinic

7. Triclinic

• We will only consider the cubic and hexagonal systems

Cubic Crystal Systems

• Simple Cubic (CUB)

• Body Centred Cubic (BCC)

• Face Centred Cubic (FCC)

Unit cell and Coordination Number • Unit cell - Smallest three-dimensional (3D) repeating unit in a

crystal structure.

• Coordination number is the number of closest neighbours a particle has in a crystal structure.

Simple Cubic • Orderly rows and stacks, with the spheres in one layer sitting

directly on top of those in the previous layer so that all layers are identical

• Each sphere is touched by 6 neighbours – 4 in its own layer, 1 above and 1 below.

• Coordination number = 6

Body-Centred Cubic • Unit cell: Simple cubic with one particle in the centre

• Instead of stacking one layer on top of the other, one layer is offset to fill the spaces (holes) in the layer above or below.

• Arrangement is a-b-a, where layer b is offset.

• Each sphere is touched by 8 neighbours – 4 above and 4 below.

Face-Centred Cubic • Each unit cell contains one additional sphere in the centre of

each face, as well as the eight at each corner.

• Each sphere is touched by 12 neighbours- 6 in the same layer, 3 above and 3 below.

• Coordination number = 12

Hexagonal Closest Packed

• Alternating layers: a-b-a, where each layer has a hexagonal arrangement and is offset to fill depressions in the other layer.

• Each sphere is touched by 12 neighbours- 6 in the same layer, 3 above and 3 below.

• Coordination number = 12

Hexagonal Closest Packed

Co-ordination number

Cubic Crystal System Coordination Number

Simple Cubic 6

Body-Centred Cubic 8

Face-Centred Cubic 12

Hexagonal Closest Packed

12

• Coordination number is the number of closest neighbours a particle has in a crystal structure.

• It is the number of atoms a given atom is in contact with.

Unit Cells • The number of atoms in a unit cell can be found by adding all

the portions of atoms found within the unit cell.

Cubic Crystal System Coordination Number # Atoms per Unit Cell

Simple Cubic 6 1

Body-Centred Cubic 8 2

Face-Centred Cubic 12 4

Hexagonal Closest Packed

12 2