Chapter 6

• What is ionic bonding?


  The electrostatic attraction between positive and negative ions.
• What is the structure of ionic compounds?

  Oppositely charged ions attract each other STRONGLY

  Each ion atracts oppositely charges ions in ALL DIRECTIONS

  In solid state, ions build up into a GIANT IONIC LATTICE

  Ions are closely packed together in a regualr lattice arrangement
• Properties of ionic compounds?

  Ionic compounds are:

  • solids at room temperature
  • high melting points - must overcome very strong ionic bonds, electrostatic forces of attraction - depending on the ionic charge and the size of the atom (strength of the bond increases when size of atom decrease) - high charge density
  • most are soluble in polar substance (such as water)
• Electrical conductivity of ionic compounds?

  Solid state:

  • ions held in fixed positions
  • no mobile charge carriers
  • no electrical conductivity

  Molten (liquid/aqueous) state:

  • ions are free to move
  • ions become mobile charge carriers
  • electrical conductors
• Define a molecule?


  A group of atoms covalently bonded together

  • simple covalent structures
  • smallest part of a covalent compound that can exist while retaining its chemical identity
• Define covalent bonding?


  The strong electrostatic sttraction between a shaired pair of electrons and the nuclei of the bonded atoms.

  • non-metals
  • compounds of non-metallic elements
  • polyatomic ions

  Is the overlap of atomic orbitals, each containing one electron, to give a shared pair of electrons

  • covalent bond is localised - electrostatic attraction is soley between the shared pair of electrons and the nuclei of the bonded atoms
• What are the HONC rules of bonding?

  H- 1 covalent bond

  O- 2 covalent bonds

  N- 3 covalent bonds

  C- 4 covalent bonds
• What are metaloids?


  Metals touching the non-metal divide line that can act as non-metals under certain conditions
• Expansion of the octet?

  only possible from n=3 upwards

  • paired electrons may be promoted to higher energy sub-shells within the same energy level, the become unpaired electrons
  • they can then be bonded
  • only non-metals
• What is a polar molecule?

  A molecule with a positive and negative end.

  • partially charged molecule
• What is non-polar molecule?

  Electronically symmetical
• What is a dipole?

  The separation of opposite charges

  • can be temporary or permanent
  • dipoles directly opposite each other with the same strength will cancel each other out and have no effect
• Ions in water?


  Due to polarity can form strong attractions with water molecules to permit dissolving and become hydrated ions

  • new attractions form which release energy
  • breaking down giant ionic lattices requires large amounts of energy
  • for compound to dissolve =energy released > energy required

  Rules of thumb:

  • All nitrates are soluble
  • All chloried (except AgCl and PbCl2) are soluble
  • Most sulphates are soluble
• Intermolecular forces?


  Are fairly weak and largely responsible for physical properties of covalent molecules
• Permanent dipole-dipole interactions?


  Only molecules which are polar with a permanent dipole

  • are permanent
  • much stronger than London forces
  • in solid state molecules stack up into a regular structure = SIMPLE MOLECULAR LATTICE -weak intermolecular forces, strong covalent bonds, low melting and boiling points
• Hydrogen bonds?


  A special type of permanent dipole-dipole interaction containing:

  • an electronegative atom with a lone pair of electrons (O, N, F)
  • hydrogen atom attached to an electronegative atom
• Electronegativity?


  Is a measure of the ability of an atom in a molecule to attract a pair of electrons in a covalent bond to itself.

  • the higher the value, the more strongly the bonding
  • electronegativity increases up the groups and across the periods
  • flourine is the most electronegative atom
  • non-metals are more electronegative than metals (group 1 = lowest)
• Non-polar bond?


  The bonded electron pair is shared equally between the bonded atoms

  Same distance between the bonded atom

  Non-polar atoms have a similar or same eletronegativity values

  • Pure covalent bond
• Polar bond?


  The bonded electron pair is shared unequally between the bonded atoms

  Different distance, one atom is partially positive/negative

  • Polar covalent bond
• Anomalous properties of water?

  Solid (ice) is less dense than liquid (water)

  • less molecules per unit water

  Relatively high melting and boiling points

  Relatively high surface tension

  • distorted tetrahedral shape - open lattice structure

  Water has the highest boiling point because it can form extensive hydorgen bonds between its molecules
• Hydrogen bonding in DNA?

  Base pairs hydrogen bonded together

  Double helix held together by intermolecular forces

  A-T = 2 hydrogen bonds

  C-G = 3 hydrogen bonds