IMF's

IMF's

1. Dipole-Dipole

   • Polarity
     • Occurs when atoms of 2 different species form a covalent bond
     • Forms a delta positive and a delta negative, forming a polar molecule
   • Dipole-Dipole forces
     • Molecules with polar bonds may have a dipole moment
     • Dipole-Dipole forces occur between molecules that have permanent dipolees

2. Van der Waals

   • The distribution of electrons (and thus of negative charge) is changing every instant
   • This means that all the charge could go to one side of the atom, forming a temporary dipole
   • The electron distribution in one atom can affect the distribution in another atom; it can induce temporary dipoles on other atoms
     • This causes a weak force of attraction
   • Act between all atoms or molecules at all times
   • The more electrons, the stronger the force
     • Explains why boiling points of noble gasses increase as atomic numbers increase
     • Explains why boiling points of hydrocarbons increase with increased chain length

3. Hydrogen Bonding

   • When do they form?
     • You need
       • A hydrogen atom that is bonded to a very electronegative atom
         • This will produce a strong partial charge on the hydrogen atom
       • A very electronegative atom with a lone pair of electrons
         • These will be attracted to the partially charged hydrogen atom in another molecule and form a hydrogen bond
     • Only form with 3 atoms
       • Nitrogen
       • Oxygen
       • Fluorine
   • What is it?
     • Some characteristics of a dipole dipole attraction and some of a covalent bond
     • Hydrogen 'sandwhiched' between 2 very electronegative atoms
     • Need an electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom
   • Boiling points of hydrides
     • Noble gases show a gradual increase in boiling points due to van der Waals
     • Boiling points of water, hydrogen fluoride and ammonia are all higher than those of hydrides of other elements in their group
   • Importance
     • When water freezes, molecules are no longer able to move
     • Hydrogen bonds hold molecules in fixed positions
     • In order to fit into the structure, which is similar to that of diamond, the molecules are slightly less closely packed than liquid water
     • Means ice is less dense than water
     • Allows ice to float
     • Insulates ponds and enables fish to survive winter