Intermolecular Forces

Van der Waals

all atoms and molecules are attracted to each other
electrons in the charge cloud are moving very quickly
the electrons are likely to become more to one side than the other-causes a temporary dipole
can cause temporary dipoles in neighboring atoms
the two dipoles are then attracted to eachother which attracts to another dipole
domino effect
due to electrons constantly moving, dipoles are being created and destroyed all the time
eg holding iodine molecules together by weak van der Waals forces
the larger the molecule, the stronger the van der Waals forces
the closer together the molecules are, the stronger the van der Waals forces
moving down the noble gases, boiling points increase due to van der Waals forces increasing
as the alkane chain gets longer, the van der Waals are stronger so boiling points increase

1 of 5

a substance made up of molecules that have permenent dipoles will have weak electrostatic forces of attraction between the + and - on neighboring molecules
eg HCl has polar molecules due to difference in electronegativity of hydrogen and chlorine
polar liquids contain molecules with permenant dipoles
the more polar the liquid is then the stronger the electrostatic attraction

(http://img1.wikia.nocookie.net/__cb20121230181112/ellesmere-chemistry/images/b/bf/Permanent_dipole_force.GIF)

2 of 5

strongest intermolecular force-usually organic
only occurs when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
very electronegative so they draw the bonding electrons away from hydrogen
the bond is polarised
hydrogen has a high charge density because it is small
hydrogen atoms form weak bonds with lone pairs of electrons on fluorine, nitrogen or oxygen
higher boiling and melting points so need extra energy in order to break them
when water cools to form ice, molecules make more hydrogen bonds-form lattice structure
hydrogen bonds are relatively long
this means ice is less dense than water

(http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/graphics/water.gif)

3 of 5

strong covalent bonds within molecules but weak forces between the molecules
do not conduct electricity as there are no free ions or electrons to carry around the charge
low melting points due to weak forces between the molecules being easily broken
some dissolve in water depending on how polarised the molecule is

4 of 5

as you move down group 7 from HCl to HI
polarity of the molecules decrease
strength of the permanent dipole-dipole interactions increase
number of electrons in the molecules increase
strength of the induced dipole-dipole interactions increase
boiling points therefore increase
increasing the strength of the dipole-dipole has a greater effect on the boiling point than decreasing the strength of the dipole-dipole
two molecules with similar number of electrons, strength of their induced dipole-dipole will be similar
if one of the substances has molecules that are more polar, there will be stronger dipole-dipole interactions and higher boiling points

5 of 5