Chemical Bonding and Molecular Structure
12.0 Hybridisation
12.1 Types of hybridization and spatial orientation of hybrid orbitals
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
12.0 Hybridisation
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
It is the mathematically fabricated concept that is introduced to explain the geometry/shapes of the covalent molecules of polyatomic ions containing covalent bonds.
It is a process of intermixing of atomic orbitals with small difference in energy and belonging to the same atom, at the time of bonding so as to give another set of orbitals with equivalent shapes and energies.
$1.$ $sp^3$ Hybridisation:
In ground state, the electronic configuration of carbon is $1s^2$, $2s^2$, $2p^2$. It is proposed that from $2s$ orbital, being quite near in energy to $2p$ orbitals, one electron may be promoted to the vacant $2p_z$ orbital thus obtaining the excited atom.
At this stage the carbon atom undoubtedly has four half-filled orbitals and can form four bonds. In the excited atom, all the four valence shell orbitals may mix up to give four identical $sp^3$ hybrid orbitals. Each of these four $sp^3$ orbital possesses one electron and overlaps with $1s$ orbitals of four $H$ atoms thus forming four equivalent bonds in methane molecule. Due to the tetrahedral disposition of $sp^3$ hybrid orbitals, the orbital are inclined at an angle of $109^° 28’$. Thus all the $H– C– H$ angles are equal to $109^° 28’$.
Shape and formation of methane molecule
$2.$ $sp^2$ Hybridisation:
When three out of the four valence obritals of carbon atom in excited state hybridize, we have three $sp^2$ hybrid orbitals lying in a plane and inclined at an angle of $120^°$.
If $2s$ and $2p$, orbitals of the excited carbon atom are hybridized, the new orbitals lie in the $xy$ plane while the fourth pure $2pz$ orbitals lies at right angles to the hybridized orbitals with its two lobes disposed above and below the plane of hybrid orbitals. Two such carbon atoms are involved in the formation of alkenes (compounds having double bonds).
In the formation of ethene two carbon atoms (in $sp^2$ hybridization state) form one sigma bond by `head-on’ overlap of two $sp^2$ orbitals contributed one each by the two atoms. The remaining two $sp^2$ orbitals of each carbon form $\sigma$ bonds with $H$ atoms. The unhybridized $2p$, orbitals of the two carbon atoms undergo a side-wise overlap forming a $\pi$ bond.
Thus the carbon to carbon double bond in ethene is made of one $\sigma$ bond and one $\pi$ bond. Since the energy of a $\pi$ bond is less than that of a $\sigma$ bond, the two bonds constituting the ethene molecule are not identical in strength. The molecule is a planar one.
Orbital model of ethane molecule
