Chemistry Notes Topic 2 - Intermolecular Forces
Intermolecular forces:
● They are the forces between molecules.
● They are very weak, much weaker than covalent, ionic or metallic bonds.
● There are 3 types resulting from these interactions; London forces, Permanent
dipoles, Hydrogen bonds.
London forces:
All atoms and molecules form London forces (also called instantaneous dipole-induced
dipole bonds).
They cause all atoms and molecules to be attracted to each other.
● Electrons in an atom move quickly and are likely to be more to one side than the
other, and this creates a temporary dipole in the atom.
● This dipole can induce another temporary dipole in the opposite direction on a
neighbouring atom. The two dipoles are then attracted to each other.
● The second dipole can then induce another dipole in a third atom, and so on.
● The electrons are constantly moving so dipoles are being created and destroyed all
the time, and overall the atoms are attracted to each other.
Properties of London forces:
● London forces can hold molecules in a lattice - simple molecular structure simply
means covalent molecules held together in a weak lattice by London forces.
● Liquids and solids with stronger London forces will have higher melting/boiling points,
as more energy will be needed to overcome the stronger intermolecular forces.
● More electrons = more London forces
● (Not all London forces are the same strength. Larger molecules or molecules
with greater surface areas, for example, will have stronger London forces)
Permanent dipoles:
● Polar molecules have permanent dipole-permanent dipole bonds.
● The positive and negative charges on polar molecules cause weak electrostatic
forces of attraction between molecules.
● These are known as permanent dipole-permanent dipole bonds.
● They can happen as well as London forces, so molecules that can form permanent
dipole-permanent dipole bonds and London forces will generally have higher
melting/boiling points.
Hydrogen bonds:
● This is the strongest type of intermolecular force.
● It only happens when hydrogen is covalently bonded to fluorine, nitrogen or
oxygen.
● F, N and O are very electronegative so they draw the bonding electrons away from
the H atom.
● The bond is very polarised and hydrogen has a very high charge density (because
it's so small) so the hydrogen atoms form weak bonds with lone pairs of
electrons on the F, N or O atoms.
Intermolecular forces:
● They are the forces between molecules.
● They are very weak, much weaker than covalent, ionic or metallic bonds.
● There are 3 types resulting from these interactions; London forces, Permanent
dipoles, Hydrogen bonds.
London forces:
All atoms and molecules form London forces (also called instantaneous dipole-induced
dipole bonds).
They cause all atoms and molecules to be attracted to each other.
● Electrons in an atom move quickly and are likely to be more to one side than the
other, and this creates a temporary dipole in the atom.
● This dipole can induce another temporary dipole in the opposite direction on a
neighbouring atom. The two dipoles are then attracted to each other.
● The second dipole can then induce another dipole in a third atom, and so on.
● The electrons are constantly moving so dipoles are being created and destroyed all
the time, and overall the atoms are attracted to each other.
Properties of London forces:
● London forces can hold molecules in a lattice - simple molecular structure simply
means covalent molecules held together in a weak lattice by London forces.
● Liquids and solids with stronger London forces will have higher melting/boiling points,
as more energy will be needed to overcome the stronger intermolecular forces.
● More electrons = more London forces
● (Not all London forces are the same strength. Larger molecules or molecules
with greater surface areas, for example, will have stronger London forces)
Permanent dipoles:
● Polar molecules have permanent dipole-permanent dipole bonds.
● The positive and negative charges on polar molecules cause weak electrostatic
forces of attraction between molecules.
● These are known as permanent dipole-permanent dipole bonds.
● They can happen as well as London forces, so molecules that can form permanent
dipole-permanent dipole bonds and London forces will generally have higher
melting/boiling points.
Hydrogen bonds:
● This is the strongest type of intermolecular force.
● It only happens when hydrogen is covalently bonded to fluorine, nitrogen or
oxygen.
● F, N and O are very electronegative so they draw the bonding electrons away from
the H atom.
● The bond is very polarised and hydrogen has a very high charge density (because
it's so small) so the hydrogen atoms form weak bonds with lone pairs of
electrons on the F, N or O atoms.
