Atomic and Molecular Orbital
The concept of covalent bond is not easy to understand. The phenomenon of sharing of electrons and its relation with the stability of molecule is explained by two theories discussed below.
Valence Bond Theory:
This theory was proposed by London. In this theory, atomic orbital relation shapes even after the formation of molecules. e.g. In the formation of the molecule the two hydrogen atoms approaches, there is orbital overlap with each other. Due to this reason, electron density is now concentrated in the region of overlap between two nuclei. The electrons now under the attractive influence of both the nuclei. Thus attractive force held the atoms of hydrogen atoms together to form the molecules.
Thus covalent bond depends on the overlapping of orbital and its strength is roughly proportional to the extent of overlap. Greater the extant of overlapping, greater the strength of covalent bond.
b) Molecular Bond Theory:
This theory was developed by Hund and Millikan. In this theory, it is assumed that atomic orbitals are not retained, after the molecule is formed. Molecules consider being a system in which nuclei are at definite distance and its own energy levels called molecular orbitals. All electrons in the molecules are moving under the influence of all nuclei. Electrons of inner shell does not involved in bonding therefore, remains unaffected and retain their positions around their own nuclei. These electrons are called non bonding electrons. Molecular orbitals are filled in a sequence and each orbital contain only two
electrons according to the Pauli`s Exclusion Principle.
Due to the wave properties of electrons the two orbitals can combine in two ways and form two orbitals. In, one molecular orbital, the charge density of electrons is concentrated in between the two nuclei. This orbital is of low energy and is called “Bonding Molecular Orbital”. The bond forming electrons will reside in the orbital. In other molecular orbital is such that there is an empty space between the two nuclei as they are now exposed. This orbital is of high energy and is called “Anti Bonding Molecular Orbital”.