Johannes Diderik van der Waals was a Dutch Physicist born in Leiden.  He went on to be educated in Leiden University.  Van der Waals was interested primarily in thermal dynamics.  He developed a theory of corresponding states on the continuity of the liquid and gaseous states of matter.  These were expressed in the van der Waals equation.

 

Van der Waals also studied the attractive forces holding the atoms of molecules together.  Molecules can attract each other at moderate distances and repel each other at close range.  There attractive forces are collectively called “van der Waals forces”.  They are relatively weak forces that attract neutral (uncharged) molecules to each other in gases, liquefied and solidified gases, and almost all organic liquids and solids.  These weak forces, much weaker than chemical bonds are disrupted by random thermal motion around room temperature.  The forces operate only when molecules pass very close to each other during collisions or near misses.

 

Intermolecular forces play a vital role in life even though we may not realise it.  For example water wouldn’t condense into solid or liquid forms if its molecules didn’t attract each other.  Van der Waals forces include all molecular forces that act between electrically neutral molecules.

 

PERMANENT FORCES

 

These forces occur when the interacting molecules contain groups or regions that are permanently electron rich (i.e. have a high electron density) or electron deficient.

 

 

 

 

 

 

 

 

 

 

 

The electron rich region on the chlorine atom is coloured blue while the electron deficient hydrogen atoms are shown in red.  The molecules appear to align when they pass close to each other because the positive end of one molecule is attracted to the negative end of the other.  The dotted red line indicates the formation of a weak intermolecular attraction during a close encounter. 

 

When the molecules have a distinctly positive end and a negative end, the permanent force is referred to as a dipole-dipole attraction.  Weaker permanent forces can act between any molecules with polar bonds e.g. the oxygen atoms in CO are electron rich while the carbon atom in the centre is electron deficient.  This means one oxygen atom of a CO molecule can be attracted to the carbon of another molecule during very close encounters.  A permanent dipole-permanent dipole is about one hundredth the force of a covalent bond.

 

HYDROGEN BONDS

 

Hydrogen bonds are abnormally strong dipole-dipole attractions.  They are the strongest type of intermolecular force. A hydrogen bond is about one tenth the strength of a covalent bond.

 

The large difference in electro negativity (ability to attract electrons) between a hydrogen and either a nitrogen, oxygen, or fluorine atom means that very polar bonds are formed.  This in turn gives the molecules strong, permanent dipole.  When a bonded electronegative atom pulls electrons away from the hydrogen atom, the positive charge that results is tightly concentrated.  The hydrogen is intensely attracted to small, electron-rich O, N, and F atoms on other molecules. This attraction set up between the positive end of the hydrogen atom and he negative end of the other (either O, N, and F) is called hydrogen bonding.