Secondary, van der Waals, or physical bonds are weak in comparison to the primary van der Waals bond or chemical ones; bonding energies are typically on the order of only 10 kJ/mol (0.1 eV/atom). Secondary bonding exists between virtually all atoms or molecules, but its presence may be obscured if any of the three primary bonding types ispresent. Secondary bonding is evidenced for the inert gases, which have stable elec- tron structures, and, in addition, between molecules in molecular structures that are covalently bonded.