Module I: The Solid State

Module H: The Solid StateOct 29 Review: The Solid StateParticles are held in an ordered arrangement. Stronger Inter-Molecular Forces than liquid or gas phases.

SolidsSolids can be classified as:Amorphous No ordered long-range structure e.g. rubber

Crystalline - Ordered arrangement over a long range. Can be subdivided into various groups: Ionic CrystalsMolecular Crystals Giant Covalent NetworksMetallic Crystals

Ionic CrystalsIons held together by electrostatic forces e.g. NaCl, LiCl

Molecular CrystalsSeparate molecules held together by intermolecular forces e.g. sucrose and ice

Giant Covalent NetworksAtoms 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 SolidIntermolecular ForcePropertiesExamplesIonicIon-ion forcesBrittle, hard, high-meltingNaCl, KBr, MgCl2MolecularDispersion forces, dipole-dipole forces, hydrogen bondsSoft, low-melting, non-conductingH2O, Br2, CO2, CH4Giant Covalent NetworkCovalent BondsHard, high-meltingC (diamond), SiO2MetallicMetallic BondsVariable hardness and melting point, conductingNa, Zn, Cu, FeCrystal systems. Particles pack together so that they can be as close as possible and maximise their interactions.Seven major crystal systems:CubicHexagonalTetragonalTrigonal Rhombic/OrthorhombicRhombicMonoclinicTriclinic

We will only consider the cubic and hexagonal systems

Cubic Crystal SystemsSimple Cubic (CUB)Body Centred Cubic (BCC)Face Centred Cubic (FCC)Unit cell and Coordination NumberUnit 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 CubicOrderly rows and stacks, with the spheres in one layer sitting directly on top of those in the previous layer so that all layers are identicalEach sphere is touched by 6 neighbours 4 in its own layer, 1 above and 1 below.Coordination number = 6

Body-Centred CubicUnit cell: Simple cubic with one particle in the centreInstead 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 numberCubic Crystal SystemCoordination NumberSimple Cubic6Body-Centred Cubic8Face-Centred Cubic 12Hexagonal Closest Packed12Coordination 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 SystemCoordination Number # Atoms per Unit CellSimple Cubic61Body-Centred Cubic82Face-Centred Cubic 124Hexagonal Closest Packed122