Topic 2 - Bonding, Structure and properties of matter

• Covalent bond


  A shared pair of electrons between atoms
• Covalent bonds


  • Occur in non-metallic elements
  • Occur in compounds of non-metals
• Forming a covalent bond between chlorine atoms

  1. Electron from each atom is shared

  2. Each chlorine atom now has 8 electrons in the outer shell
• Covalent bonds are very strong
• Simple molecules


  • Methane
  • Water
  • Hydrogen chloride
• Simple molecules


  • Contain a relatively small number of non-metal atoms joined together by covalent bonds
  • Have no overall electrical charge, so they cannot conduct electricity
  • Are usually liquids or gases at room temperature due to weak intermolecular forces
• Size of simple molecules

  Larger molecules have stronger intermolecular forces and higher melting/boiling points
• Group 7 elements


  • Fluorine and chlorine are gases
  • Bromine is a liquid
  • Iodine is a solid
• Giant covalent structures


  • All atoms are linked by strong covalent bonds
  • Require breaking of covalent bonds to melt or boil
  • Have very high melting and boiling points
• Diamond


  • A form of carbon with a giant, rigid covalent structure
  • Each carbon atom forms four strong covalent bonds with other carbon atoms
  • Very hard substance with a very high melting point
  • Does not conduct electricity
• Graphite


  • Another form of carbon with a giant covalent structure
  • Each carbon atom forms three covalent bonds with other carbon atoms
  • Has a layered, hexagonal structure
  • Layers held together by weak intermolecular forces, allowing them to slide past each other
  • One electron from each carbon atom is delocalised, allowing graphite to conduct heat and electricity
• Silicon dioxide (silica)


  • Has a lattice structure similar to diamond
  • Each oxygen atom is joined to two silicon atoms
  • Each silicon atom is joined to four oxygen atoms
• Nanoparticles


  • Contain only a few hundred atoms
  • Can combine to form nanostructures
  • Have different properties compared to the same materials in bulk, e.g. more sensitive to light, heat and magnetism
  • Atoms can be placed in exact positions, so smaller quantities are needed to achieve required properties/effects
• Uses of nanoparticles


  • Sun creams
  • New drug delivery systems
  • Synthetic skin for burn victims
  • Computers and technology
  • Catalysts for fuel cells
  • Stronger and lighter construction materials
  • New cosmetics and deodorants
  • Fabrics that prevent the growth of bacteria
• Concerns remain that nanoparticles could get into and damage human cells or cause problems in the environment
• Metallic bonding


  • Occurs in metals and alloys
  • Involves a giant structure with delocalised electrons in the outer shell
  • Positive ions are held together by electrostatic attraction to the delocalised electrons
• Properties of metals


  • Metallic bonds are very strong, so most metals have high melting and boiling points
  • Delocalised electrons can move freely, making metals good thermal and electrical conductors
  • Particles have a regular arrangement, allowing the layers to slide over each other easily, making metals ductile and malleable
• Alloys


  • Mixtures that contain a metal and at least one other element
  • Added elements disrupt the regular arrangement of metal atoms, making alloys harder than pure metals
• Steel


  • An alloy made from iron
  • Contains small amounts of carbon and other metals
  • Steel with high carbon content is hard and strong
  • Steel with low carbon content is soft and easily shaped
  • Stainless steel contains chromium and nickel, making it hard and resistant to corrosion
• Other useful alloys


  • Copper alloys (with silver, copper, zinc)
  • Aluminium alloys (combine low density with high strength)
  • Bronze (copper and tin)
  • Brass (copper and zinc)