Chemical bonding (VBT)

Chemical Bonding - (VBT)




Energy diagram of s-s overlap of Hydrogen molecule


Bond Length and Bond Energy
Bond Energy: The amount of energy given off or released per mole at the time of overlapping of atomic
orbitals to form a bond is termed as Bond Energy.

Bond Length: The equilibrium distance at which the forces of repulsion are balanced by forces of
stabilization/merger between two atomic nuclei is termed as Bond Length.


Why does a stronger bond have a shorter bond length?


Covalent Bond and Types

1.

Sigma Bond : Head-On Overlap

s-s overlap Ex: H2;
s-p overlap Ex: HF, HCl
p-p overlap Ex: F2, Cl2

2.

Pi Bond : Sidewise/lateral overlap

During the formation of sigma bond the extent of overlapping is more and hence sigma bond is stronger than pi
bond





Explaining Single Bonds
For molecules like hydrogen fluoride :

• The 1s orbital of H overlaps with the half-filled 2p orbital of F


25 % s-character and 75% p-character


Explaining Single Bonds
For molecules like methane:

•
•

the VB theory of hybrid orbitals is used to explain molecular shape
3
carbon forms four hybrid orbitals (sp ) by combining three 2p orbitals and a 2s orbital so that four
identical bonds can be created

3

The four sp orbitals of C overlap with the s orbitals of H to form methane. Mixing of 1 s orbital with three p orbitals.



33 % s-character and 67 % p-character


Explaining Double Bonds
For bond formation in ethene:

•
•
•

2
one sp orbital of each carbon overlaps to form a σ bond between the carbons
2
two sp orbitals of each carbon overlap with the 1s orbitals of the hydrogens to form σ bonds
the lobes of the 2p orbitals of each carbon overlap above and below the plane to form a pi ( π) bond

2
The three sp orbitals of C overlap with the s orbitals of H to form ethene. Mixing of 1 s orbital with two p orbitals.




50 % s-character and 50 % p-character


Explaining Triple Bonds

For molecules like ethyne:

•
•
•

the linear structure is explained by formation of 2 sp hybrid orbitals for each carbon (a 2s orbital + a 2p orbital)
sigma bonds form from overlap between sp of each carbon and between sp of carbons and 1s of hydrogens
two pi bonds form from overlap of the two 2p orbitals of each carbon

The two sp orbitals of C overlap with the s orbitals of H to form ethyne. Mixing of 1 s orbital with one p orbital.