Post on 08-Apr-2016
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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