Unit3

• Covalent bonding


  Bonding between atoms that share electrons
• Ionic bonding


  Bonding between a metal and a non-metal where the metal loses electrons to the non-metal
• Metallic bonding


  Bonding between metal atoms where the outer electrons are delocalised and shared between the atoms
• Potassium has one electron
• Fluorine has 7 electrons
• Ionic bond


  Forms when a metal transfers electrons to a non-metal
• Covalent bond


  Forms when atoms share electrons
• Weak forces


  Van der Waals forces and hydrogen bonds
• Covalent bonds like HCl have high melting and boiling points
• Ionic compounds are usually solid at room temperature and are non-conductive when solid
• Ionic compounds are conductive when molten as the ions are free to move
• Non-metals are usually poor conductors of electricity
• The charge of an ion depends on the number of electrons lost or gained
• Metallic bonding


  Involves a lattice of positive metal ions with delocalised electrons in between
• Graphite


  Has layers of carbon atoms with weak forces between the layers, allowing the layers to slide over each other
• Diamond


  Has a tetrahedral arrangement of carbon atoms with strong covalent bonds, making it very hard and with a high melting point
• Fullerenes


  Made of balls and cages of carbon atoms
• Uses of fullerenes


  • Lubricants
  • Drug delivery
• The electrical properties of graphite, graphene and carbon nanotubes are due to their delocalised electrons
• Diamond does not conduct electricity as it has no free electrons
• Composites


  Combinations of materials with different physical or chemical properties
• Kevlar


  A polymer with strong covalent bonds arranged in one direction, giving it high strength, low weight and flexibility
• Stress = Force / Cross-sectional area
• Qualitative analysis determines if a substance is present
• Quantitative analysis determines the concentration of a substance
• Semi-quantitative analysis gives a rough idea of how much of a substance is present
• Flame tests can identify the presence of certain metal ions
• Chromatography can separate the components of a mixture
• Rf = Distance travelled by substance / Distance travelled by solvent
• Photosynthesis uses carbon dioxide, water and light energy to produce glucose and oxygen
• Respiration uses glucose and oxygen to produce carbon dioxide, water and energy
• Factors limiting the rate of photosynthesis are light intensity, temperature and carbon dioxide concentration
• Intensive farming uses high-yield crops, fertilisers, pesticides and efficient machinery
• Organic farming avoids synthetic chemicals and uses biological pest control
• Hydroponics grows plants without soil, using a mineral nutrient solution