The concept of hybridization came into being because valence bond theory
was not able to explain the bonding phenomenon in a number of molecules. The
best example is compounds of carbon like CH4. In CH4,
carbon forms four covalent bonds. According to valence bond theory, each
covalent bond is formed by the overlap of two singly filled atomic orbitals.
The electronic configuration of carbon does not allow the formation of
four bonds as can be seen below.
There are only two singly filled orbitals for carbon. The covalency of
carbon is four and thus it was suggested that one of the electron from 2s
orbital jumps or gets excited to 2pz orbital thus creating four
singly filled orbitals, i.e.
Now if these orbitals combine as such, the four bonds formed in CH4
would be of two different energies because the combining capacity of 2s and 2p
orbitals is different. But it was seen experimentally that all the four bonds
in CH4 are of the same strength and thus of the same energy. This means that
the four singly filled orbitals are of the same energy. This is possible only when
they mix together to redistribute the total energy and take up the average
energy. Thus “The
mixing of the atomic orbitals, which are of slightly different energy, of an
atom so as to redistribute the energy in such a way that formation of new
orbitals of equal energy takes place is called hybridization” and the
new orbitals thus formed are called hybrid orbitals.
The type of hybridization deduces the shape of a particular molecule.
Type of Hybridisation
sp Hybridisation:
In this hybridization, one s orbital intermix with one p orbital and form two hybrid orbitals named sp orbital. sp hybrid orbitals are arranged at 180 degrees and form a linear shape.
sp2 Hybridisation:
one orbital of s subshell and two orbitals of p subshell with comparable energy are mixed to each other and form three hybrid orbitals of equal energy and shape. These hybrid orbitals are named sp2 hybrid orbital. The hybrid orbitals are formed a trigonal shape with an angle of 120 degrees therefore the resultant shape of the molecule becomes trigonal.
sp3 hybridisation
when one orbital of s subshell is intermix with three orbitals of p subshell then four hybrid orbitals are formed with same shape and energy. each orbital is known as sp3 hybrid orbital and joined to each other at 109.5 degrees with tetrahedral geometry. The molecule containing sp3 hybrid central atom is tetrahedral in shape.
sp3d hybridisation
This hybridization is result of the intermixing of five orbitals (one "s" orbital, three "p" orbitals, and one "d" orbital ) of subshells of comparable energy. In this hybridization, five hybrid orbitals are formed with similar energy and the same shape. These orbitals form a shape of trigonal bipyramidal. The bond angles are found 120 degrees and 90 degrees.
sp3d2 hybridisation
sp3d2 hybridisation is also known as octahedral hybridisation. this hibridisation is a result of intermixing of six orbitals of s, p, and d subshell respectively. The bond angle between hybrid orbitals is 90 degrees.
sp3d3 hybridisation
The molecule of iodine heptaflouride is pentagonal bipyramidal because the Iodine atom is sp3d3 hybridised in this molecule.
A summary of hybridization type shape relationship
is given below:
Four simple steps for determination of Hybridisation
Step 1: Draw the lewis dot structure (Electron dot structure)
Example
Step 2: Count the number of lone pair and bond pairs at the central atom
Step 3: Find the hybridization from the following table as per the number of lone pair and bond pair.
Step 4: Conclusion
The carbon atom of the above molecule is sp3 hybridized and the bond angle is 109.5 degrees.