Forces acting between molecules

• Three types of intermolecular bonds

  • Van der waals forces (all atoms and molecules)
  • Dipole-dipole forces (certain types of molecules)
  • Hydrogen bonding (certain types of molecules)
• Dipole moments

  • Polarity is the property of a particular bond
  • Polar bonds might have a dipole-dipole
  • In molecules with more than one polar bond, the effects may cancel and have no dipole moment, but may also add up and reinforce each other (depends on the shape)
• Dipole-dipole forces act between molecules that have permanent dipoles.
• Van der waals

  • Positive and negative molecules all have very weak electrostatic forces, even if an atoms overall charge is neutral.
  • Act between all atoms and or molecules at all times
  • Increases with an increase in electrons
  • (explains why longer chain hydrocarbons increase in boiling point)
• Hydrogen bonding

  • Need a very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom (H2O is polar)
  • Much stronger than dipole-dipole bonds but considerably weaker than covalent
  • Only N, O, and F are electronegative enough to form hydrogen bonds
  • Always linear
• Boiling points of the hydrides

  • Noble gasses show a gradual increase as only van der waals forces acting
• Structure and density of ice:

  • Water, hydrogen bonds break and reform easily as molecules are moving around
  • Ice, hydrogen bonds no longer free to move around and hydrogen bonds hold the molecules in fixed positions
  • To be in this structure, they are more loosely packed together and therefore are more dense