Answer
Zero energy refers to two chlorine atoms which are not at all attracted or repelled by each other. This occurs when the atoms are infinitely far apart, where the electrostatic force is completely negligible.
Work Step by Step
The energy approaches zero as the distance between the two chlorine atoms increases. Because there is an inverse relationship between the distance between the atoms and the electrostatic force, the force (and the energy) decrease as the distance between the atoms increases. Thus, zero energy would occur when the two chlorine atoms are so far apart that the electrostatic force between them is completely negligible.
