1. An ionic lattice: a giant regular repeating pattern of alternating positive and negative ions in 3D. The packing structure of the ions depends on the relative sizes of the ions and their respective charges. This regular lattice is responsible for crystal shapes. E.g. NaCl makes cubic crystals. Ionic Crystals Introduction

2. Ionic Crystals Properties High melting/boiling points: A large amount of energy is required to break electrostatic forces. Solubility? Ions are highly attracted to polar water molecules. Less soluble in non-polar solvents. Brittle: Stress on the crystal can bring ions of the same charge into close proximity- cleavage planes. Relative strength? Electrical conductors? Only when molten or in solution.

3. Small molecules of (held together by strong covalent bonds) are held together by weak intermolecular bonds in a regular 3D arrangement. Low melting/boiling points: intermolecular bonds are easily broken. Solubility? Polarity of the molecule? Often soft crystals Electrical conductors? No, there are no charged particles (electrons or ions). Simple Molecular Crystals

4. Large numbers of atoms are linked in a regular 3D arrangement by covalent bonds. High melting/boiling points: strong covalent bonds are not easily broken. Solubility? Never!! Electrical conductors? No, there are no charged particles (electrons or ions). There is one exception… Giant Molecular Crystals Examples: diamond, graphite, graphene

5. Diamond Each carbon atom forms 4 covalent bonds with other carbon atoms (with bond angles 109.5°) Very high melting point (over 3700 K) Very hard material Electric conductivity? No, there are no charged particles to carry the charge Diamond and graphite are both made of elemental carbon only. They are polymorphs or allotropes of carbon

6. Graphite Each carbon atom forms 3 covalent bonds with other carbon atoms (with bond angles 120°) Very high melting point Soft material (layers) Electric conductivity? Yes. Each carbon atom has a spare electron in a p orbital, that is not part of the 3 single covalent bonds. These electrons are delocalised http://www.youtube.com/watch?v=dTSnnlITsVg&list=TLYGpn 0EweQ-yzCOavakSCEKIP3rXE847t

7. A lattice of positively charged ions, embedded in a sea of delocalised electrons High melting/boiling points: A large amount of energy is required to break electrostatic forces. Strength? This depends again on the charge of the metal ion, and the size of the metal ion Solubility? No Electrical conductors? Yes Metallic Crystals