Chemistry · Chemical Bonding · NEET
When the two atoms are far apart, they do not feel each other, so energy is taken as zero. As they come closer, NEW attractive forces start: the nucleus of atom A pulls the electron of atom B, and the nucleus of B pulls the electron of A. These pulls bring the atoms together. Experimentally, this new attraction is stronger than the new repulsion, so the system becomes more stable and the potential energy goes DOWN (becomes negative).
There are TWO attractive forces: (1) nucleus of A with electron of B, and (2) nucleus of B with electron of A. There are TWO repulsive forces: (1) nucleus of A with nucleus of B, and (2) electron of A with electron of B. Simple rule: opposite charges attract (nucleus–electron), like charges repel (nucleus–nucleus, electron–electron). Attraction pulls atoms together; repulsion pushes them apart.
The minimum is the most stable point. Here the net attraction exactly balances the net repulsion, so there is no more pull to move closer or apart. The potential energy is the lowest possible. At this distance the two H atoms are said to be bonded and a stable H2 molecule is formed. For H2 this happens at 74 pm, which is the bond length.
Bond length is the equilibrium distance between the nuclei of the two bonded atoms. For H2 the energy is minimum at an internuclear distance of 74 pm, so this is the natural resting distance of the two nuclei. That is why 74 pm is the bond length of H2. The depth of the valley (about 435.8 kJ/mol) is the bond enthalpy, the energy released.
If you push the atoms closer than 74 pm, the two positive nuclei come very near each other and the nucleus–nucleus repulsion grows very fast. The electron–electron repulsion also rises. Now repulsion beats attraction, so the potential energy rises steeply. This is why the curve turns upward on the left side of the minimum, forming a valley shape.
Bond formation is EXOTHERMIC (energy is released). As the atoms fall into the energy valley, the system loses energy to the surroundings, equal to the depth of the well (435.8 kJ/mol for H2). To break the bond you must put the SAME energy back, so bond breaking is endothermic. NEET tip: lower energy means more stable, and a stable molecule releases energy when it forms.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The bond length of H2 is 74 pm (the internuclear distance at minimum energy) and its bond dissociation enthalpy is about 435.8 kJ/mol, which is the depth of the energy well.
Valence Bond Theory (VBT), given by Heitler and London, explains it. It considers the overlap of the 1s orbitals of the two H atoms and the balance of attractive and repulsive forces as they approach.
Yes. Lower potential energy means greater stability. The H2 molecule at the minimum of the curve is more stable than two separate H atoms, which is why energy is released when the bond forms.
To the right (larger distance), the atoms feel weaker attraction, so energy slowly rises back toward zero. Far apart, the atoms are independent and the energy is zero (no bond).
