Engineering Chemistry S Datta

20 ENGINEERING CHEMISTRY
(v) N 2
molecule: p-p overlapping to form σ and π bonds
p y
p z
p y
p z
p y
p z
p y
p z
-bond
p x
-bond
Fig. 2.7 σ and π bonds in N 2
molecule.
(a) Overlapping of two p x
orbitals along the axes leads to the formation of a sigma bond.
(b) Lateral overlapping of two p y
and p z
orbitals leads to the formation of two π bonds.
Hybridization: Carbon has electronic configuration: 1s 2 2s 2 2p 2 , but in CH 4
, C atoms form
four identical bonds with H-atom with equal energy. This can be explained only by the Concept
of hybridization. If we consider that C forms four hybrid orbitals of equal energies out of 2s
and 2p orbitals and then these hybrid orbitals overlap with s-orbitals of H-atoms along the axes,
then bond angle and bond energies in CH 4
molecule can satisfactorily be explained as follows:
C-atom (ground state) ⇒
1s 2s 2p
C-atom (excited state) ⇒
Now this one s-orbital and three p-orbitals of a carbon atom mix to give four hybrid
orbitals of equal energies, the axes of hybrid orbitals are directed towards the vertices of a
regular tetrahedron (Fig. 2.8).
+ + +
+ + +
s p x
sp 3 sp 3 sp 3 sp 3
hybrid orbitals
p y
p z
sp 3
109° 28
sp 3
sp 3 C
sp 3
Fig. 2.8 sp 3 hybridised C atom.
Now these four sp 3 hybrid orbitals overlap along the axis with s-orbitals of four H-atoms
to form a CH 4
molecule forming four covalent bonds of same energies.