Valence Bond (VB) Theory: This theory has following postulates: Covalent bonds are formed by overlapping of atomic orbitals. Electron density is high in the overlapped area and is thus responsible for holding atoms together. Only half filled atomic orbitals take part in covalent bond formation. The half filled atomic orbitals must contain electrons withValence Bond (VB) Theory Notes | Trick to identify Sigma bond and Pi bond – Chemistry