VSEPR THEORY
Geometry of molecule
- It is the relative arrangement of bonded atoms in a molecule.
- The way in which the bonded atoms direct themselves around the central atom is explained on the basis of this theory.
- This theory was given by Sid wick and Favell.
- In this only the valence electrons participate in bond formation.
- The bonded atoms around central metal atom will have repulsions between them
- Therefore, they will arrange themselves in such way that the repulsion is minimum.
In BF3 the arrangement is like it is shown below:
![](http://free-education.in/wp-content/uploads/2021/03/image-321.png)
![](http://free-education.in/wp-content/uploads/2021/03/image-322.png)
![](http://free-education.in/wp-content/uploads/2021/03/image-323.png)
Characteristics
- Number of hybrid orbitals is equal to number of atomic orbitals that combine.
- The hybrid orbitals are always equal in shape and energy.
- The hybrid orbitals are more effective in forming bonds as compared to pure atomic orbitals.
- The hybrid orbitals are directed towards specific directions in space.
- The type of hybridization gives us the shape of molecule
Now there is a question that can all participate in hybridization
Conditions of hybridization
- The only valence orbitals participate.
- The atomic orbitals that participate should have almost same energy.
- Promotion is not always necessary.
- The unpaired as well as fully filled orbitals can also participate.
Types of hybridization
- sp hybridization: In this one s and one p orbital combines as shown below.
![](http://free-education.in/wp-content/uploads/2021/03/image-324.png)
![](http://free-education.in/wp-content/uploads/2021/03/image-325.png)
Valence bond approach of covalent bond
VSEPR theory does not tell us about bond parameters like directional nature, bond angle, repulsions etc.
To explain we have new theory:
- Valence bond approach
- Molecular orbital theory
- Valence bond theory
Assumptions:
- According to this, the atom retains their identity even after bonding.
- Bond is formed due to interaction of valence electrons only.
- Only the valence electrons lose their identity whereas inner electrons do not participate.
- Stability of bond depends upon amount of energy released
- The molecule has minimum energy at a specific distance called inter-nuclear distance and at that the bond formation takes place.
Overlapping: sigma and pi bond
Overlapping can be defined as partial merging of bonded orbitals .More is the overlapping, stronger is the bond formed.
Types of overlapping
- Head to head
- Sidewise
Accordingly, the bond formed is sigma or pi bond.
Sigma bond:
- In this head to head overlapping occurs.
- More is the overlapping region more stable is the bond.
- The bond is stronger.
- This bond can exist independently.
Pi bond:
- It is formed by side wise overlapping of orbitals.
- It is not too strong as in this overlapping region is less.
- It is weaker bond as compared to sigma bond.
- It can’t exist independently, it exist along with sigma bond.
Let’s make Sigma bond :
![](http://free-education.in/wp-content/uploads/2021/03/image-326.png)
![](http://free-education.in/wp-content/uploads/2021/03/image-327.png)
- Molecular orbital theory
It was developed by F.Hund and R.S Mullikan in 1932.
The features of this theory are:
- The electrons in a molecule are present in various molecular orbital as the electrons of atom are present in different various shells.
- The atomic orbitals of comparable energies and proper symmetry combine to form molecular orbitals.
- In molecular orbital electrons are in influence of two or more nuclei.
- The number of molecular orbitals formed is equal to number of atomic orbitals that combine.
- The two orbitals formed due to combination are: Bonding and Anti -bonding.
- The Bonding molecular orbital has lower energy and greater stability than Anti -bonding.
- The electron probability distribution around group of nuclei is given by molecular orbital.
- The molecular orbitals are filled in accordance to Aufbau principle, Pauli’s principle and Hund’s rule.
The linear combination of atomic orbitals to form molecular orbital takes place only if:
- The combining orbitals must have same energy.
- The combining orbitals must have same symmetry.
- The combining orbitals must overlap to maximum extent.
The order to fill molecular orbital is:
![](http://free-education.in/wp-content/uploads/2021/03/image-328.png)
![](http://free-education.in/wp-content/uploads/2021/03/image-329.png)
- We can find the bond order : That is
Bond Order 1 (single bond)
Bond Order 2 (double bond)
Bond Order 3 (triple bond)
( c) Tell us about the type of bond :
- Magnetic character :
If unpaired electrons – paramagnetic
If no unpaired electrons – dimagnetic