In chemisorption, a strong 'chemical bond' is formed between the adsorbate atom or molecule and the substrate. In this case, the adsorption energy, E a , of the adatom is likely to be a good fraction of the sublimation energy of the substrate, and it could be more. Physisorption is weaker, and is often described as implying that no chemical interaction is present. This can't really be true, because if there were no attractive interaction, then the atom wouldn't stay on the surface for any measurable time- it would simply bounce back into the vapor. A better distinction is that in physisorption, the energy of interaction is largely due to the van der Waals force.
Typical systems are rare gases on layer compounds and other similar systems.
One can see that these energies are comparable to the sublimation energies of rare gas solids, as given in Lecture 1, Table 1. Adsorption of molecules often proceeds in two stages. A first, precursor stage , has all the characteristics of physisorption, but this state is metastable. In this state the molecule may re-evaporate, or it may stay on the surface long enough to transform irreversibly into a chemisorbed state. The adsorption energies for the precursor phase are similar to phyisorption of rare gases, but may contain additional contributions from the dipole, quadrupole, etc moments of the molecules.