Grade 11 - Chemistry - Chemical Bonding Moecular Structure - Hybridisation



In many cases the formation of covalently bonded molecules cannot be simply explained on the basis of overlapping of atomic orbitals only.
Example; carbon on interacting with hydrogen forms simple hydrocarbon methane. However, the simple overlapping cannot explain the formation of methane molecule. T give explanation it is necessary to introduce the concept of hybridization.

i) This concept was introduced by Pauling.
ii) Hyridisation is defined as the intermixing of dissimilar orbitals of the same atom but having slightly different energies to form equal number of hybrid orbitals of same energy and identical shapes.

Characteristics of Hybridisation:

i) Orbitals having almost equal energies combine to form hybrid orbitals.
ii) Electron does not take part in hybridization.
iii) The number of combining atomic orbitals give the same number of hybrid orbitals.
iv) In the excited state, paired electron if any is unpaired and promoted to next available vacant orbital(s).
v) Both half-filled orbitals or fully filled orbitals of close energy can involve in hybridization. S-orbital of equivalent energy always participate in the process of hybridization.
vi) Most of the hybrid orbitals are similar in shape, size and energy.
vii) Pi-bond is formed by overlapping of simple atomic orbitals, not hybrid orbitals.
viii) Hybridisation takes place only on interaction with other atom.
ix) The extent of overlapping of hybrid orbitals is more than overlapping of simple atomic orbitals.
x) Hybridised orbitals having lower energy than simple atomic orbitals.


Types of Hybridisation:

sp hybridization

a) s- and p- orbitals in the valency shell mix together to form two equivalent hybrid orbitals of same shape and energy. It is known as sp hybridization.
b) They are linear in geometry witb an angle of 1800
c) Hybrid orbitals having 50% s-character and 50% p-character, i.e. maximum s-character possible in hybrid orbital.
Now chlorine pzatomic orbitals overlap with hybrid orbitals forming BeCl2 molecule. Similarly HgCl2 C2H2,etc. can be explained through sp hybridization.

Sp2 Hybridisation:

a) One s-orbital and two p-orbitals of an atom mix together to form three equivalent hybrid orbitals, is known as sp2 hybridisation.
b) This hybrid orbital are planar in geometry and angle between them is 120degree.
c) Sp2hybrid orbital possessing 33.3% s-character and 66.6% p-character.
Now fluorine pz overlaps with sp2 hybrid orbitals forming BF3. Its geometry is planar triangular.

Sp3 hybridisation:

a) In this case one s-orbital and three p-orbitals of an atom mix together to form four hybrid orbitals of same shape, size and energy. These orbitals are known as sp3 hybridised orbitals.
b) The hybridized sp3 orbitals get the shape of regular tetrahedron with angle of 109degree28/ with one another.
c) In this hybrid orbital , 25% s-character and 75% p-character.Formation of Methane molecule can be best explained by sp3 hybridisation.