Case 1 (upper curve, antibonding)

1. Electrons are spinning in the same direction and repel each other.
2. As the moveable H atom is moved towards the fixed H atom, the first interaction, between each atom’s electrons, is repulsion, and the potential energy rises.
3. Since the electrons repel, they are pushed out of the internuclear region.
4. As the atoms are moved even closer, the next interaction, between the positively charged nuclei, is also repulsion. The potential energy rises rapidly to infinity as the internuclear distance approaches zero.

Case 2 (lower curve, bonding)

1. Electrons are spinning in the opposite direction and do not repel each other significantly.
2. As the moveable H atom is moved towards the fixed H atom, the first interaction, between each atom’s electrons, is insignificant, since the electrons repel each other only very slightly.
3. But the each electron is attracted to both positively charged nuclei and is swept between the nuclei forming an electron pair, the covalent bond..
4. The negatively charged electrons, which avoid each other by the direction of spin, pull in the positively charged nuclei and the potential energy drops.
5. As the atoms are moved even closer, the next interaction, between the positively charged nuclei, is minimized at first by the shielding effect of the electrons. But eventually, at very small internuclear distances, repulsion increases rapidly causing the potential energy to shift directions and turn up towards infinity.

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