Tuesday, January 29, 2013

VSEPR Theory ( Valance Shell Electron Pair Repulsion )

  1. The electron pair in the valence shell around the central atom of a molecule repel each other and tend to orient in space so as to minimize the repulsion and maximize the distance between them.
  2. There are two types of valence shell electron pairs .
  • Bond pairs
  • Lone pairs
Bond pairs are shared 2 atoms and are attracted by 2 nuclei. Hence they occupy less space and cause less repulsion.
Lone pairs are not involved in bond formation and are in attraction with only one nucleus. hence they occupy more space. As a result the lone pair causes more repulsion.
The order of repulsion between the different types of electron pairs is as follows:
Lone pairs - Lone pairs > Lone pair - Bond pair > Bond pair - Bond pair.

Note :
     Bond pairs are usually represented by a solid line, where as the lone pairs are represented by a lobe with 2 electrons.

  1. In VSEPR theory the multiple bonds are treated as if they where single bond. The electron pairs in multiple bonds are treated collectively as a super pair. The repulsion caused by bonds increases with increase in the number of bonded pairs between two atoms. ie : The triple bond causes more repulsion than the double bond which turn causes more repulsion than single bond. 
  2. The shape of molecules can be predicted from the number and types of valence shell electron pairs around the central atom. When the valence shells of central atom contains only bond pairs. The molecules assumes symmetrical geometry due to even repulsion between them. However the symmetry disorted when there are also lone pairs along with bond pairs due to uneven repulsion forces.
  3. Primary and secondary effects on bond angle and shape :
  •  The bond angle decreases due to the presence of lone pairs, which cause more repulsion on the bond pairs and as a result the bond pairs tend to come closer.
  • The repulsion between electron pairs increase with increase in electro negativity of central atom and hence the bond angle is increases. The bond pairs are closer and thus by shortening the distance between them, which turn to increase the repulsion.Hence the bond tends to move away from each other.
The bond pairs tend to move away from each other since the distance between them is shortened as they are more localized on more electronegativity central atom.
However the bond angle decreases when the electronegativity of the ligand atoms are more than that of central atom. There is increase in distance between bond pairs since they are closer to ligand atoms. Due to this they tend to move closer resulting in the decrease in bond angle.
The bond pairs tend to come closer since the distance between them is increases as they are more localized on more electronegative ligand atoms.
The bond angle decrease with increase in the size of central atom.
On smaller central atoms the bond pairs are closer and hence tend to move away from each other so as to minimize repulsion. Hence bond angle will be more.
On bigger atoms, the bond pairs are distant from each other and hence there is less repulsion. They tend to move closer and thus by decrease the bond angle. However the bond angle in the size of the ligand atoms, which surrounded the central atom.
There is less repulsion between smaller ligand atoms and they can move closer to each other and decrease the bond angle.
There is more repulsion between bigger ligand atoms and hence they tend to move away from each other and the bond angle is increases.
The bond angles are also changed when multiple bonds are present.It is due to uneven repulsion.
  1. when there are two or more resonance structures, the VSEPR theory is applicable to any of such contributing structure. 

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