chapter 6 - shapes of molecules, and intermolecular forces

• tetrahedral shape


  4 bonded pairs

  109.5 bond angle
• pyramidal 


  3 bonding pairs

  1 lone pair

  107 bond angle
• non-linear

  2 bonding pairs

  2 lone pairs

  104.5
• the bond angle is reduced by 2.5 for each lone pair
• CO2 has a linear shape
• octahedral


  6 bonding pairs

  90 bond angle
• ammonium has a tetrahydral shape
• nitrate has a trigonal planar shape
• trigonal planar


  3 bonding pairs

  120 bond angle
• sulfate ions have a tetrahyral shape
• the attraction of a bonded atom for the pair of electrons in a covalent bond is called electronegativity
• electronegativity trends


  across the periodic table - electronegativity increases

  down the periodic table - electronegativity decreases
• Why does electronegativity increase across the periodic table?

  • nuclear charge increases
  • the atomic radius decreases
• if the electronegativity difference in a molecule is 0, the bond is covalent

  if it is 0 to 1.8, the bond is polar covalent

  if it is larger than 1.8, the bond is ionic
• a pure covalent bond is where the bonded atoms come from the same element
• a polar covalent bond is when the bonded atoms have different electronegativity values
• if a molecule is symmetrical, the dipoles cancel each other out, and the molecule is not polar
• three main types of intermolecular forces

  • induced dipole-dipole interactions (london)
  • permanent dipole-dipole interactions
  • hydrogen bonding
• london forces are the weakest

  permanent dipole-dipole interactions are stronger

  hydrogen bonds are strongest of intermolecular
• how a london bond is formed

  • movement of electrons produces a changing dipole in a molecule
  • at any instant, and instantaneous dipole will exist, but its position is constantly shifting
  • the instantaneous dipole induces a dipole on a neighbouring molecule
  • the induced dipole induces further dipoles on neighbouring molecules, which then attract one another
• the strength of induced dipole-dipole interactions

  the more electrons in a molecule:

  • the larger the instantaneous and induced dipole
  • the greater the induced dipole-dipole interactions
  • the stronger the attractive forces between molecules
• permanent dipole-dipole interactions act between the permanent dipoles in different polar molecules
• in solid state, sample molecules form a simple molecular lattice
• properties of simple molecular substances

  • low melting and boiling points (weak intermolecular forces)
  • non polar substances soluble in non-polar solvents, they tend to be insoluble in polar solvents
  • no mobile charged particles, so they do not conduct electricity
• non-polar simple molecular compound dissolving in non-polar solvent

  intermolecular forces form between the molecules and the solvent

  these interactions weaken the intermolecular forces in the simple molecular lattice, they break and the compound dissolves
• solubility of polar simple molecular substances

  depends on the strength of the dipole, and can be hard to predict
• hydrogen bonds can from between an electronegative atom, with a lone pair of electrons (e.g. oxygen, nitrogen or fluorine) and hydrogen
• properties of water

  • ice less dense than liquid (hydrogen bonds hold water molecules further apart in ice)
  • water has a relatively high melting and boiling point, because of the strong hydrogen bonds between molecules