Final answer:
The function U(x) = -C6/x^6 represents an attractive force between two hydrogen atoms when they are far apart, as it is energetically favorable for the atoms to approach each other and minimize their potential energy. The attraction dominates until reaching a minimum energy state at a bond length of 74 pm. However, if the atoms get too close, the force becomes repulsive due to the repulsion between like charges.
Step-by-step explanation:
The potential energy of a pair of hydrogen atoms can tell us about the nature of the interactions between them. Given the potential energy function U(x) = -C6/x6, where C6 is a positive constant, we can determine the force between the atoms. The negative sign signifies an attractive force because as the distance between atoms x increases, the potential energy approaches zero from below, indicating that the atoms will tend to move closer to each other to lower the potential energy of the system. When atoms are at a very large distance, the attractive and repulsive forces are negligible, but as they approach each other, the attractive forces initially dominate.
The attractive force continues to dominate until the system reaches a point where the potential energy is at its lowest, which corresponds to a stable bond distance. However, if these atoms come too close beyond this point, the repulsive electron-electron and proton-proton forces increase the potential energy, indicating a repulsive force. The bond length for H2 is 74 picometers (pm), and the energy at this distance is the minimum possible, which corresponds to the maximum stability of the molecules formed.