Bonding

• Give the meaning of the term electronegativity


  The power of an atom to attract an electron pair in a covalent bond
• Explain how permanent dipole-dipole forces arise between molecules

  The difference in electronegativity leads to bond polarity.

  There is attraction between delta+ on one molecule and delta- on another.

  (Dipoles don't cancel so the molecule has an overall permanent dipole)
• Ionic bonding involves:

  Electrostatic forces of attraction between oppositely charged ions in a lattice
• A coordinate bond (dative covalent) contains:

  a shared pair of electrons with both electrons supplied by one atom
• Metallic bonding:

  Attraction between delocalised electrons and positive ions arranged in a lattice
• Explain why the melting point of sodium fluoride is high (2):

  Electrostatic forces of attraction between oppositely charged ions Na+ and F- so more energy needed to overcome forces
• Explain why a fluoride ion is larger than a sodium ion (2):

  Fluoride ion has lower nuclear charge so there is weaker attraction between the nucleus and outer electrons
• Ionic structure: Sodium chloride

  Strong electrostatic forces of attraction.

  Has high melting and boiling point.

  When molten or in solution, can conduct electricity as ions are free to move and carry flow of charge.
• Metallic structure: E.g. Magnesium or Aluminium

  Often good conductors, sea of delocalised electrons can carry a flow of charge.

  Metals are malleable as layers of positive ions are able to slide over one another.

  Electrostatic forces of attraction between positive ions and delocalised electrons are strong, more energy needed to overcome so high melting points and nearly always solid at room temp.
• Simple Molecular Structure: Iodine

  Consist of covalently bonded molecules held together by weak VDW forces.

  VDW forces are weak so less energy needed to overcome so lower melting point.

  Very poor conductors as their structure contains no charged particles.
• Macromolecular Structure: Diamond or Graphiteor Silicon Dioxide

  Covalently bonded into giant lattice.

  Each atom has multiple covalent bonds, which are strong, giving the substance high melting points.

  Strength of covalent lattice makes the structure rigid.

  Graphite can conduct electricity as electrons not used in bonding can move between layers
• Simple Molecular: Ice

  Has hydrogen bonding so higher melting point
• Linear Shape - E.g. BeCl2

  Bond pairs: 2

  Lone pairs: 0

  Bond angle: 180 degrees
• V-Shaped shape - H2O

  Bond pairs: 2

  Lone pairs: 2

  Bond angle: 104.5 degrees
• Trigonal Planar Shape - E.g. BF3


  Bond pairs: 3

  Lone pairs: 0

  Bond angle: 120 degrees
• Trigonal Pyramidal - E.g. NH3


  Bond pairs: 3

  Lone pairs: 1

  Bond angle: 107 degrees
• Tetrahedral Shape - E.g. CH4


  Bond pairs: 4

  Lone pairs: 0

  Bond angle: 109.5 degrees
• Trigonal Bipyramidal - E.g. PF5 or PCl5

  Bond pairs: 5

  Lone pairs: 0

  Bond angle: 90 and 120 degrees
• Octahedral Shape - E.g. SF6

  Bond pairs: 6

  Lone pairs: 0

  Bond angle: 90 degrees
• Which polymer has hydrogen bonding between the polymer chains?

  Kevlar
• Explain why pentan-2-ol has a higher m.p than pent-1-ene (3)

  Pentan-2-ol has stronger intermolecular forces whilst pent-1-ene has VDW forces only

  Pentan-2-ol also has hydrogen bonding so more energy is needed to overcome forces
• Explain why silicon dioxide has a higher melting point than sulfur trioxide (4)

  There are strong covalent bonds between atoms in SiO2 but weak IMFs between molecules. Covalent bonds are therefore harder to break so the giant covalent SiO2 has a higher melting point than SO3.
• Explain, in terms of crystal structure/bonding, why silicon oxide has a higher melting point than phosphorus oxide

  SiO2 is a giant covalent structure held together by many strong covalent bonds - more energy needed to overcome

  P4O10 is a simple covalent molecule with VDW forces between molecules in the solid are weak and easily overcome