6. shapes of molecules and intermolecular forces

• Intermolecular forces - weak interactions between dipoles of different molecules
• Induced dipole-dipole interactions (London forces)

  • even noble gases can be liquified and solidified. They are single atoms so no covalent or ionic bonding
  • to exist in liquid o solid state there must be forces between these atoms
  • All atoms and molecules are able to exert attractive forces known as induced dipole-dipole forces or London (dispersion) forces
• intermolecular forces determine physical properties of covalent molecules
• covalent bonds determine the identity and chemical reactions of a molecule
• • All atoms and molecules are able to exert attractive forces known as induced dipole-dipole forces or London (dispersion) forces
• London forces

  • electrons in an atom or molecule are constantly moving
  • A temporary uneven distribution of electrons can occur for an instant
  • This creates an instantaneous dipole
  • The instantaneous dipole induces a dipole in its neighbours leading to an attraction
  • Each dipole can induce many others
• Non-polar molecules only have London forces between their molecules. As molecules get larger, the number of electrons increases so size of the induced dipoles also get larger, giving stronger forces. 
• As the strength of the London forces increases, more energy is needed to overcome them, so the melting and boiling point increases.
• Permanent dipole-dipole interactions

  • formed between molecules which are polar
  • they exert full time forces so are stronger than London forces
  • opposite dipoles attract one another e.g HCl = H+ - Cl-
  • As the strength of the permanent dipole-dipole interactions increases, more energy is needed to overcome the additional forces, so the melting and boiling point increases.
• Electronegativity - The ability of an atom in a molecule to attract the pair of electrons in a covalent bond to itself
• polar covalent bond - a shared pair of electrons where the electron pair is not shared equally between the two bonded atoms
• dipole - a charge separation across a bond with one atom having a slight positive charge and the other having a slight negative charge
• How does an induced dipole form?

  Fluctuation in the electron density around a molecule creates an instantaneous dipole in a molecule. The instantaneous dipole induces a dipole in a neighbouring molecule
• Molecular shapes from 2 regions of electron density:

  • Shape = linear
  • Angle = 180
• Molecular shapes from 3 regions of electron density:

  • Shape = trigonal planar
  • Angle = 120
• Molecular shapes from 4 regions of electron density:

  • Shape = tetrahedral
  • Angle = 109.5
• Molecular shapes from 5 regions of electron density:

  • Shape = trigonal bipyramidal
  • Angle = 90 and 120
• Molecular shapes from 6 regions of electron density:

  • Shape = octahedral
  • Angle = 90
• Methane:

  • bonded pairs = 4
  • lone pairs = 0
  • shape = tetrahedral
  • angle = 109.5
• ammonia:

  • bonded pairs = 3
  • lone pairs = 1
  • shape = pyramidal
  • angle = 107
• Water:

  • bonded pairs = 2
  • lone pairs = 2
  • shape = non-linear
  • angle = 104.5