Explain the Valence Bond Theory $(VBT)$.

(N/A) The $VBT$ was introduced by Heitler and London $(1927)$ and further developed by Pauling and others. It addresses the limitations of the Lewis approach and $VSEPR$ theory.
Key postulates of $VBT$:
$1$. Formation of a covalent bond: It occurs due to the overlapping of half-filled atomic orbitals of the participating atoms.
$2$. Energy minimization: As two atoms approach each other,attractive and repulsive forces act between them. At a specific distance,the system reaches a state of minimum potential energy,resulting in a stable covalent bond. For example,in $H_2$,the energy decreases by $435.8 \ kJ \ mol^{-1}$.
$3$. Extent of overlapping: The strength of a covalent bond is directly proportional to the extent of overlapping of the atomic orbitals.
$4$. Directional properties: The geometry of a molecule is determined by the directional nature of the overlapping atomic orbitals (e.g.,tetrahedral geometry in $CH_4$).
$5$. Types of bonds: Overlapping along the internuclear axis results in a $\sigma$ bond,while lateral (perpendicular) overlapping results in a $\pi$ bond.
$6$. Hybridization: Atomic orbitals undergo hybridization to form equivalent hybrid orbitals,which then overlap to form bonds.