VB or Valence Bond Theory:
The VB was developed in order to explain chemical bonding using the method of quantum mechanics. This theory primarily focuses on the formation of individual bonds from the atomic orbitals of the participating atoms during the formation of a molecule.
-------------
VB theory predicts covalent bond formation between atoms when they have half-filled valence atomic orbitals, each containing a single unpaired electron.
These atomic orbitals overlap, so electrons have the highest probability of being within the bond region. Both atoms then share the single unpaired electrons to form weakly coupled orbitals.
The two atomic orbitals do not need to be the same as each other.
The theory helps explain bond formation in cases where a Lewis structure can't describe real behavior.