Ionic : electrostatic force of attraction between oppositely charged ions Sodium chloride Magnesium oxide Covalent : shared pair of electrons Simple molecular: With intermolecular forces (van der Waals, permanent dipoles, hydrogen bonds) between molecules Iodine Ice Carbon dioxide Water Methane Covalent : shared pair of electrons Macromolecular: giant molecular structures. Diamond Graphite Silicon dioxide Silicon Metallic: electrostatic force of attraction between the metal positive ions and the delocalised electrons Magnesium, Sodium (all metals) Only use the words molecules and intermolecular forces when talking about simple molecular substances Giant metallic lattice Property Ionic Molecular (simple) Macromolecular Metallic boiling and melting points high- because of giant lattice of ions with strong electrostatic forces between oppositely charged ions. low- because of weak intermolecular forces between molecules (specify type e.g van der waals/hydrogen bond) high- because of many strong covalent bonds in macromolecular structure. Take a lot of energy to break the many strong bonds high- strong electrostatic forces between positive ions and sea of delocalised electrons Solubility in water Generally good generally poor insoluble insoluble conductivity when solid poor: ions can’t move/ fixed in lattice poor: no ions to conduct and electrons are localised (fixed in place) diamond and sand: poor, because electrons can’t move (localised) graphite: good as free delocalised electrons between layers good: delocalised electrons can move through structure conductivity when molten good: ions can move poor: no ions poor (good) general description crystalline solids mostly gases and liquids solids shiny metal Malleable as the positive ions in the lattice are all identical. So the planes of ions can slide easily over one another -attractive forces in the lattice are the same whichever ions are adjacent. Four types of crystal structure: ionic, metallic, molecular and giant covalent (macromolecular). You should be able to draw the following diagrams or describe the structure in words to show the four different types of crystal. You should also be able to explain the properties of these solids. The tables earlier in the revision guide explain these properties. Ionic: sodium chloride Giant Ionic lattice showing alternate Na+ and Clions Metallic: magnesium or sodium Giant metallic lattice showing close packing magnesium ions Molecular: Iodine Regular arrangement of I2 molecules held together by weak van der Waals forces Molecular: Ice The molecules are held further apart than in liquid water and this explains the lower density of ice Macromolecular: diamond Tetrahedral arrangement of carbon atoms. 4 covalent bonds per atom Macromolecular: Graphite Planar arrangement of carbon atoms in layers. 3 covalent bonds per atom in each layer. 4th outer electron per atom is delocalised. Delocalised electrons between layers. Both these macromolecular structures have very high melting points because of strong covalent forces in the giant structure. It takes a lot of energy to break the many strong covalent bonds
188.8.131.52 Bonding and physical properties
The four types of crystal structure:
• macromolecular (giant covalent)
The structures of the following crystals as examples of these four types of crystal structure:
• sodium chloride.
Students should be able to:
• relate the melting point and conductivity of materials to the type of structure and the bonding present
• explain the energy changes associated with changes of state
• draw diagrams to represent these structures involving specified numbers of particles.