Q.According to Valence Bond theory, a bond between two atoms is formed when

Valence Bond (VB) theory, originating in Heitler and London's wave-mechanical treatment of the hydrogen molecule and developed further by Pauling and Slater, explains covalent bond formation as the direct RESULT of atomic orbital overlap.

The H₂ picture. Starting with two hydrogen atoms at infinite separation (potential energy taken as zero), bringing them closer introduces new attractive forces (each nucleus to the OTHER atom's electron) and new repulsive forces (nucleus-nucleus, electron-electron). Initially, as the atoms approach, attraction dominates and the potential energy falls; it reaches a MINIMUM at the point of maximum orbital overlap — this minimum defines the bond length (74 pm for H₂) and the energy released there (436 kJ mol⁻¹) is the bond energy. Push the atoms closer still, and repulsion takes over, sharply raising the energy again.

Salient features (the working rules), generalised from this picture:

1. A bond forms when a HALF-FILLED orbital on one atom overlaps with a half-filled orbital on another.
2. The resulting overlap region is occupied by two electrons of OPPOSITE spin.
3. Bond STRENGTH scales with the EXTENT of overlap — greater overlap, more energy released, stronger bond.
4. Every orbital except the spherical s orbital has a preferred spatial direction, so overlap occurs along whichever direction MAXIMISES it. …