Hybridisation:
To explain the shapes of polyatomic molecules, the atomic orbitals combine to form new set of equivalent orbitals known as hybrid orbitals. Instead of pure atomic orbitals, these hybrid orbitals are used in bond formation. This process is known as hybridization. Therefore, “the process of intermixing of orbitals of slightly different energies so as to redistribute their energies resulting in the formation of new set of orbitals of equivalent energies and shape”.
The new orbitals formed as a result of hybridisation are called hybrid or hybridised orbitals. Thus, to form four equivalent bonds one 2s and three 2p-orbitals of carbon hybridise and form four new orbitals called sp3 (pronounced as s-p3) hybrid orbitals.
Characteristics of Hybridization:
- The number of hybridised orbitals formed is equal to the number of the orbitals that get hybridised.
- The hybridised orbitals are always equivalent in energy and shape.
- The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.
- The hybrid orbitals are directed in space in some preferred directions to have stable arrangement. Therefore, the type of hybridisation indicates the geometry of the molecule.
Chemical Bonding:
“A chemical bond, in general is the force of attraction that binds the constituent particles ( e.g. atoms, molecules, ions) together in various chemical species”.
Atoms enter into chemical combination to attain a state of lowest energy and maximum stability. Based upon this idea, the formation of a number of different kinds of bonds like ionic, covalent, co-ordinate, metallic, hydrogen bond etc.
The concept of covalent bond formation based on sharing of electrons (Lewis concept), was a purely qualitative approach. The above basis could not offer a satisfactory answer regarding the force which holds together the constituent particles. In order to offer a reasonable explanation, the modern scientist proposed two theories, namely—
- Valence Bond Theory (VBT)
- Molecular orbital Theory (MOB).
Valence Bond Theory (VBT):
The salient features of valence bond theory are as follows—
i) According to VBT combining atoms maintain their identity even in the molecules and the bond is formed by the interaction of half filled atomic orbital belonging to the valence shell of the participating atoms.
ii) The overlapping atomic orbitals must have their electron with opposite spins.
iii) The strength of the bond depends upon the extent of overlapping. The grater the overlap, stronger the bond.
iv) As a result of overlapping and therefore, pairing of electrons, energy is released and hence the system acquires a state of lower energy.
v) The valencey of an element is equal to the number of half filled orbitals in the valence shell of its atom.
This theory was able to explain successfully the formation of various molecules like H2,F2, O2, NH3, H2O etc. and their shape yet it has many limitations.
Limitations of VBT: – Followings are the limitations of valence bond theory—
i) Combining atom do not lose their identity seems faulty. It is due to the fact that the electron cloud of an atom is affected by the presence of its neighbouring atoms.
ii) It hardly explains the paramagnetic character of oxygen molecule.
iii) It fails to explain the formation of metallic bond and bonding in electron deficient compound like B2H6.
iv) It could not explain the formation of species like H2+, He2+ which contain odd number of electrons.