Grade 11 - Chemistry - Chemical Bonding Moecular Structure - Hybridisation

Part-III

Sp3d hybridization:

a)This hybridization involves one s-orbital three p-orbitals and one d-orbital of an atom mix together to form five hybrid orbitals of same shape, size and energy. This hybrid orbital is known as sp3d hybrid orbital.
b)Without any distortion the shape of this orbital is trigonal bi-pyramidal(TBP). Formation of PCl5 is best explained with such hybridization of phosphorus.

Sp3d2 Hybridisation:

a) This hybridization involves one s-orbital, three p-orbitals and Two d-orbitals.
b)This set of six orbitals are directed to the corners of a regular Octahedron. This is best represented by the formation of SF6.

Sp3d3 Hybridisation:

a)sp3d3hybrid orbitals are formed in an atom by intermixing of one s-orbital, three p-orbitals and three d-orbitals of an atom.
b)It is best explained on the basis of formation of IF7 molecule.This seven atomic orbitals hybridizes to sp3d3hybrid orbitals. Now fluorine pzorbital overlapps the hybrid orbitals forming a molecule IF7. This molecule having pentagonal bipyramidal geometry.

Geometry and Shapes of Molecules(VSEPR Theory):

a) It is best explained on the basis of hybridization along with Valency shell electron pair repulsion theory.
b) We have already discussed hybridization. In this the direction of overlap of orbitals gives direction of bonds.
c) The VSEPR theory was put forward by Sidwick and Powell. It is very helpful to predict the geometry of the molecule.Arrangement of bonds around the central atom is best explained with the help of repulsions of electron pairs around the central atom.
d) In polyatomic molecules containng 3 or more atoms, one is considered as central atom to which other atoms are bonded.
e) The geometry of a molecule depends upon the total number of bonded and non-bonded(lone pair) electron pairs around the central atom and their repulsions due to shapes and sizes.
f) When the central atom is bonded to similar atoms and not having any lone pair, in such case the repulsion is also of same magnitude and provides a symmetrical shape of the molecule.
g) The central atom bonded to atoms of different sizes or surrounded by bond pair , the geometry of the molecule becomes distorted. In this case the magnitude of repulsions differ one from other, and is represented as below:
lp- lp> lp – bp>bp - bp

Now we applying the concept of hybridization and as well as VSEPR theory to certain selective molecules to establish the final shape and geometry of the molecule concerned.

i) Diatomic molecule is linear shape.
ii) Polyatomic molecules:

a) Symmetrical molecules:
In which the central atom bonded to same atoms and not having any lone pair or unpaired electrons, the shape becomes symmetrical due to same magnitude of repulsion due to bp – bp. Example CH4 molecule.

b) Unsymmetrical molecules:
If the central atom contains one or more lone pairs, the geometry of the molecule becomes unsymmetrical due to Lone pair-bond pair or lone pair-lone pair repulsion.e.g.In ammonia one lone pair and in water two lone pairs present. Both the molecules are Sp3 hybridized as in CH4, but they are some what distorted tetrahedral due to lp –bp repulsion which is greater than bp – bp repulsion in NH3,thereby angle is reduced to 1070. In H2O lp –lp repulsion is far greater than bp – bp repulsion, hence angle is reduced to 104.50.

c) In polyatomic unsymmetrical molecules, if the central element only differs, then the angle increases with increase in electrnegativity of the central atom.
example.

d) In the polyatomic molecules, when the central element is same but the side atoms different, then depending on the increase in electronegativity of the side atoms the angle decreases.

In the other word we can say as the size of side atoms increases the bond angle also increases.
Shapes of Molecules containing bond pairs only:

i) Shape of BeF2 molecule:
The hybridization of Be in BeF2 is SpHybridization. The geometry of the milecule is linear i.e.180degree. On the basis of VSEPR there is only bp – bp repulsion, so they can go maximum to 180degree.
ii)Shape of BF3 molecule:
The hybridisation of central element is Sp2,hence having planar triangular structure with F- B – F angle 1200 .On the basis of VSEPR theory the three bond pair is arranged as such to give angle 120 degree,hence a planar structure.