topic 2

• Types of compounds


  • Substances in which 2 or more elements are chemically combined
• Types of strong chemical bonds

  • Ionic
  • Covalent
  • Metallic
• Ionic bonds


  • Particles are oppositely charged ions
  • Occurs in compounds formed from metals combined with non-metals
• Covalent bonds


  • Particles are atoms which share pairs of electrons
  • Occurs in most non-metallic elements and in compounds of non-metals
• Metallic bonds


  • Particles are atoms which share delocalised electrons
  • Occurs in metallic elements and alloys
• Ionic bonding


  1. Metal + Non-metal: electrons in the outer shell of the metal atom are transferred

  2. Metal atoms lose electrons to become positively charged ions

  3. Non-metal atoms gain electrons to become negatively charged ions

  4. An ion is an atom that has lost or gained electron(s)
• Ions produced by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 gains full outer shell of electrons. So they have the same electronic structure as a noble gas (Group 0 element)
• Electron transfer during the formation of an ionic compound can be represented by a dot and cross diagram
• Ionic compounds are giant structures of ions held together by strong electrostatic forces of attraction between oppositely charged ions. Since the structure is in 3D, the forces act in every direction. An example is sodium chloride (salt)
• Covalent bonding


  • Atoms share one or more pairs of electrons
  • Small molecules like HCl, H2, O2, Cl2, NH3, CH4 have strong covalent bonds within their molecules
  • Polymers are large covalently bonded molecules
  • Giant covalent structures consist of many atoms covalently bonded in a lattice structure like diamond, silicon dioxide
• Metallic bonding


  • Bonding in a metal consists of positive ions and delocalised electrons arranged in a regular pattern
  • Delocalised electron system consists of the electrons 'lost' from the atoms to form positive ions
  • Delocalised electrons are free to move through the structure
  • Delocalised electrons are shared through the structure so metallic bonds are strong
• The three states of matter

  • Solid
  • Liquid
  • Gas
• Melting and freezing


  Take place at the melting point
• Boiling and condensing


  Take place at the boiling point
• Particle theory


  1. Can help to explain melting, boiling, freezing, and condensing

  2. The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance

  3. The nature of the particles involved depends on the type of bonding and the structure of the substance

  4. The stronger the forces between the particles, the higher the melting point and boiling point of the substance
• State symbols


  • Solid (s), Liquid (l), Gas (g), Aqueous solutions (aq)
• Properties of ionic compounds

  • Regular structures (giant ionic lattices) with strong electrostatic forces of attraction in all directions between oppositely charged ions
  • High melting and boiling points due to the many strong bonds
  • Conduct electricity when melted or dissolved in water because ions are free to move and carry current, but cannot conduct electricity when solid as ions are fixed in place
• Properties of small molecules

  • Usually gases or liquids with low boiling and melting points
  • Weak intermolecular forces between the molecules, broken in boiling or melting, not the covalent bonds
  • Intermolecular forces increase with the size of the molecules
• Characteristics of substances with low boiling and melting points

  • Usually gases or liquids
  • Have weak intermolecular forces between the molecules
  • Intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
  • Substances consisting of small molecules do not conduct electricity as they do not have an overall electric charge
• Polymers


  • Have very large molecules
  • Atoms in the polymer molecules are linked to other atoms by strong covalent bonds
  • Intermolecular forces between polymer molecules are relatively strong, making these substances solids at room temperature
• Giant Covalent Structures


  • Substances consisting of giant covalent structures are solids with very high melting points
  • All atoms in these structures are linked to other atoms by strong covalent bonds
  • These bonds must be overcome to melt or boil these substances
• Giant Covalent Structures


  • diamond
  • graphite
  • silicon dioxide (silica)
• Properties of metals and alloys


  • Metals have giant structures of atoms with strong metallic bonding
  • Most metals have high melting and boiling points
  • Layers of atoms in metals can slide over each other, allowing metals to be bent and shaped
  • Alloys are made from 2 or more different types of metals
  • Different sized atoms in alloys distort the layers in the structure, making it harder for them to slide over each other, resulting in alloys being harder than pure metals
• Metals as conductors


  • Good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal
  • Good conductors of thermal energy because energy is transferred by the delocalised electrons
• In diamond, each carbon is joined to 4 other carbons covalently
• Diamond


  • Very hard, has a very high melting point, does not conduct electricity
• In graphite, each carbon is covalently bonded to 3 other carbons, forming layers of hexagonal rings which have no covalent bonds between the layers
• Graphite


  • Layers can slide over each other due to no covalent bonds between the layers, soft and slippery, one electron from each carbon atom is delocalised
• One electron from each carbon atom is delocalised


  Makes graphite similar to metals because of its delocalised electrons, can conduct electricity unlike Diamond
• Graphene is a single layer of graphite
• Graphene


  • Useful in electronics and composites, very strong because atoms within its layers are tightly bonded, elastic because the planes of atoms can flex relatively easily
• Graphite


  Has properties that make it useful in electronics and composites
• Graphene


  Very strong because atoms within its layers are very tightly bonded and it is also elastic because the planes of atoms can flex relatively easily without the atoms breaking apart
• Types of carbon molecules

  • Fullerenes
  • Carbon nanotubes
• Fullerenes


  Molecules of carbon atoms with hollow shapes based on hexagonal rings of carbon atoms, but they may also contain rings with five or seven carbon atoms
• The first fullerene to be discovered was Buckminsterfullerene (C60), which has a spherical shape
• Carbon nanotubes


  Cylindrical fullerenes with very high length to diameter ratios
• Properties of carbon nanotubes

  • Can be used as lubricants, to deliver drugs in the body and catalysts
  • Can be used for reinforcing materials, for example tennis rackets
• Carbon nanotubes


  A type of fullerene (form of carbon with a cylindrical shape) with a very high length to diameter ratio; can be used in electronics, energy storage, and composite materials