Final answer:
Hydrogen nuclei repel each other due to electrostatic repulsion between positively charged protons. An optimal distance allows electron-nuclei attraction to counteract this repulsion and form a stable molecular structure, but when too close, repulsion dominates and the system becomes high-energy and unstable.
Step-by-step explanation:
The nuclei of hydrogen atoms repel each other because of electrostatic repulsion. This is due to both nuclei possessing a positive charge; the protons in the nuclei are positive and like charges repel each other. This repulsion increases significantly as the nuclei get closer. While the attractive forces between the nuclei and their electrons can initially counteract this repulsion, there exists an optimal internuclear distance where the potential energy is at a minimum, resulting in a stable molecular structure with the lowest possible potential energy.
When hydrogen atoms approach each other, their electron clouds overlap and a covalent bond can form, lowering the potential energy of the system. However, as the nuclei get closer than the optimal internuclear distance, the repulsive forces between the positively charged nuclei overpower the attraction, and the potential energy rises drastically, leading to an unstable, high-energy situation. This is seen in the steep rise of potential energy shown in the energy diagram when nuclei are too close together.
The balance between nuclear attraction and electrical repulsion is essential in processes such as nuclear fusion, which powers stars like our Sun. Although like charges repel, getting nuclei close enough to come within the 'striking distance' of the strong nuclear force can overcome this repulsion and enable fusion, releasing energy in the process.