Final answer:
As nuclei move toward each other, the potential energy of the system initially decreases due to attractive forces, reaches a minimum at an optimal internuclear distance, then increases if they get too close due to repulsive forces.
Step-by-step explanation:
When two nuclei approach each other, the potential energy of the system changes. Initially, as they move closer, the potential energy decreases because the attractive forces between the electrons and the protons in the different nuclei dominate. There is a point where the potential energy is at its minimum, indicating the most stable configuration of the system. However, if the nuclei continue to move even closer, the repulsive forces between the positively charged nuclei become significant, leading to an increase in potential energy.
This behavior can be illustrated by a graph of potential energy as a function of internuclear distance. At great distances, the potential energy is relatively high due to the lack of interaction. As the distance decreases, the potential energy lowers due to attractive interactions reaching a minimum at an optimal internuclear distance. Past that point, repulsive forces cause a steep climb in potential energy. In the context of hydrogen atoms, this optimal distance is known as the bond distance, which is 74 pm for a hydrogen molecule (H₂).