topic 2

• Metals are composed of a three-dimensional lattice of positive ions, surrounded by a "sea" of mobile/delocalised valence electrons

  e.g. sodium and magnesium

  
• Metallic bond: the lattice of positive ions is held together by the electrostatic attraction between the positive ions and the mobile delocalised electrons. 
• Physical Properties of Metals:

  • have a shiny (lustrous) surface when pure (cleaned or freshly cut)
  • most are silver/grey in colour (except silver and gold)
  • are ductile (drawn into wires) and malleable (hammered into sheets without breaking)
  • good conductors of electricity (solid/liquid) and heat
  • high melting points
• METALS - Malleable and ductile


  when the metal lattice is placed under stress, the layers of positive ions can be pushed passed each other without significant repulsion, due to the presence of the delocalised electrons between the layers which are able to move and maintain the metallic bond.
• METALS - Good conductors

  due to the delocalised electrons which are free to move under the

  influence of an electric field
• METALS - High Melting & Boiling Points:

  A large amount of energy is required to overcome the strong electrostatic attraction between the metals cations and the delocalised

  electrons (the metallic bond)
• Ionic substances are formed when a metal reacts with a non-metal  They can also be formed when ammonia reacts with acids to produce ammonium compounds.
• Electrons are transferred (either lost or gained) to gain full outer shell/stable configuration to form positive or negative ions (cations/anions)
• Ionic solids are composed of alternating positive and negative ions arranged in a three-dimensional lattice.  Each ion is held in the lattice by strong electrostatic attractions to the oppositely charged ions surrounding it.  This attraction between the ions is called ionic bonding. 
• Physical properties of Ionic Compounds

  • the solids are very hard and brittle
  • they are non-conductors of electricity in the solid state, but good conductors when molten or when dissolved in water
  • they have high melting and boiling points.

   
• During the melting process we assume that ionic bonds are broken and the charged ions are free to move. When in aqueous solution the ions dissociate and are also free to move.
• IONIC COMPOUNDS - High melting/boiling points

  
  

  The electrostatic attractions between oppositely charged ions are strong. A large amount of energy is required to break the bonds so the melting point and boiling points are high. The greater the charge on the ions the stronger the attraction between them

  
  

  e.g. (CaO has a higher mp (2572 oC)than KF (858 oC )
• IONIC COMPOUNDS - Hard

  Bonds are strong between ions so large forces can be applied.
• IONIC COMPOUNDS - Brittle

  When lattice is distorted, like charges come together and repel and the crystal shatters. (Repulsion force)
• IONIC COMPOUNDS - Does not conduct as a solid


  Ions are in fixed positions due to strong forces so no charged particles are free to move in the solid phase. No mobile charged particles
• IONIC COMPOUNDS - Conducts in molten state


  When the particles have gained energy and ionic bonds have been broken the charged ions are now free to move and carry the current.
• IONIC COMPOUNDS - Conducts in aqueous solution if soluble


  When dissolving occurs, the positive and negative ions dissociate and are free to move and conduct electricity.
• Electron dot diagrams can be used to show the arrangement of valence electrons (outer shell electrons) in an atom, ion or molecule. 
• Octet rule: when an atom reacts to attain eight electrons in outermost shell to have a stable electron configuration
• Polyatomic ions are ions composed of two or more atoms covalently bonded together. Compounds that contain polyatomic ions have both ionic and covalent bonding.
• Ionic compounds which contain polyatomic ions have the same physical properties as other ionic compounds.
• A molecule is formed when two or more non-metal atoms bond together by sharing electrons.  
• A pair of atoms in a molecule is held together by the electrostatic attraction between the shared negative electrons and the positive nuclei of the atoms.  The two atoms are said to be held together by a covalent bond. 
• Substances that are composed of atoms covalently bonded together in small molecules (discrete molecules) are called covalent molecular substances. These discrete molecules are neutral particles. The atoms within a molecule are held together by strong covalent bonds within these substances, yet only weak intermolecular/dispersion forces between molecules.   
• Physical properties of covalent molecular substances

             -  they have low melting and boiling points

             -  they are non-conductors of electricity in solid and liquid states

             -  they form solids that are often soft and have a waxy appearance.
• Only weak intermolecular forces are broken or disturbed in physical changes. When a covalent molecular substance melts weak forces between the molecules are broken, leaving the molecules intact but able to move more freely.
• COVALENT MOLECULAR SUBSTANCES - Low melting/boiling points


  Weak intermolecular forces are broken during the melting and boiling processes. Only a small amount of energy is required to break them.
• COVALENT MOLECULAR SUBSTANCES - Soft/brittle

  Relatively weak intermolecular forces between molecules so little force is required to rearrange the molecules.
• COVALENT MOLECULAR SUBSTANCES - Does not conduct as solid or liquid


  The particles in covalent molecular substances are molecules which are neutral particles. There are no charged particles free to move.
• COVALENT MOLECULAR SUBSTANCES - Do not conduct when dissolved in water, if soluble (some exceptions to this e.g. HCl, HNO3, H2SO4)


  No charged particles free to move. The exceptions to this are substances, such as strong or weak acids and bases, which react with water and produce ions (ionisation) which are then involved in the conduction of current
• COVALENT MOLECULAR SUBSTANCES - Odour

  Substances with an odour tend to be covalent molecules. Weak intermolecular forces are easily broken to allow molecules to enter the gas phase.
• Covalent network substances are composed of a network of atoms covalently bonded together held strongly in a lattice to form a giant molecule. E.g silicon, carbon which has different allotropes – diamond, graphite
• COVALENT NETWORK SUBSTANCES - High melting/boiling points (many covalent network substances decompose rather than boil) 


  strong covalent bonds. All of the bonds present in a covalent network substance are strong covalent bonds between atoms. Large amounts of energy are required to break these bonds.
• COVALENT NETWORK SUBSTANCES - Hard


  Strong covalent bonds so large forces can be applied without breaking the bonds.
• COVALENT NETWORK SUBSTANCES - Brittle


  Lattice will break rather than distort.
• COVALENT NETWORK SUBSTANCES - Does not conduct in solid or molten phase (exception, e.g. graphite)


  No charged particles free to move. Atoms are in fixed positions and valence electrons in covalent bonds. Graphite = exception; its structure includes some free electrons.
• Graphite  - does not have all of the typical properties of a covalent network substance due to its unusual structure
• Allotropes: different crystalline forms of the same element, i.e. graphite and diamond are allotropes of carbon. Both made up of same type of atom but atoms are arranged in different ways.
• In diamond, each carbon is covalently bonded to four other carbon atoms in a tetrahedral arrangement to form a continuous three-dimensional structure (i.e. covalent network lattice).

  
• In graphite, each carbon is covalently bonded to three other carbon atoms in layers of hexagonal rings (i.e. covalent layer lattice). The fourth valence electron from each carbon atom is able to move within the layer. Between the layers there are weak forces called dispersion forces.