Final answer:
Van der Waals forces are the exception to requiring an energy barrier for bond formation, as these forces are weak and transient compared to the strong covalent or ionic bonds that need significant energy to form or break.
Step-by-step explanation:
The question is about the exception to bond formation requiring an energy barrier, which is Van der Waals forces. These are weaker forces that occur due to transient electric polarization of molecules. Unlike strong covalent or ionic bonds, Van der Waals forces do not have a significant energy barrier to form.
This is because they result from temporary shifts in electron density rather than the sharing or transferring of electrons that you see in covalent or ionic bonds, which require bond dissociation energy or lattice energy to form or break. Additionally, the bond energy for a strong covalent bond, like the C=C bond, is much higher than for these weaker forces. Therefore, when discussing the energy barrier in molecular interactions, Van der Waals forces stand out as requiring no substantial input of energy to establish.