Types of Forces
There are 3 main types of intermolecular forces, from weakest to strongest these are:
- London forces
- Permanent dipole-dipoles
- Hydrogen bonding
They are all very weak compared to covalent, ionic and metallic bonds.
London forces are found between ALL atoms and molecules.
Electrons in charge clouds are always moving, and at any one time you'll find more electrons on one side of a molecule than the other. This difference between sides causes something called a temporary dipole, (This is means there is a difference in charge across the molecule).
This temporary dipole on molecule 1 for example, can cause another dipole to form on a neighbouring molecule, for example molecule 2. The two dipoles which have formed can now be attracted to each other, (a positive side on one molecule is attracted to the negative side of another) This happens on a huge scale, with many molecules giving others dipoles. These attractions between dipoles are called London forces.
Some molecules are polar, this occurs when one atom in a molecule is more electronegative than the other, for example in hydrochloric acid (HCl). The chlorine atom is more electronegative than hydrogen, this results in the bonding electrons being pulled more towards the chlorine, making chlorine slightly more negative (because electrons are negatively charged), and thus - making the hydrogen slightly more positive. So, the bond between hydrogen and chlorine is known as a polar bond.
What happens now is in a solution full on lots of HCl, the slightly negatively charged chlorine is attracted electrostatically to the slightly positively charged hydrogen, on a different HCl molecule.
These forces of attraction are called permanent dipole-dipole interactions, and occurs in all polar molecules.
This is the strongest type of intermolecular force.
Hydrogen bonding only occurs when hydrogen is covalently bonded to either fluorine, nitrogen or oxygen. Because these three atoms are very electronegative, they pull bonding electrons away from hydrogen easily - making a very polarised bond. Because it's so polarised, and hydrogen now has a very positive charge compared to the other atom, it can form weak bonds with the lone pairs of electrons on the fluorine, nitrogen or oxygen atoms of other molecules.
For example, water contains hydrogen bonding because the oxygen in a water molecule pulls bonding electrons away from hydrogen, causing it to be positively charged. The hydrogen then makes bonds with the lone pairs of electrons on another oxygen in the water. This is also the same for ammonia (NH3), because the nitrogen has a lone pair of electrons.