Why physisorption forms multi-molecular layers, whereas chemisorption forms unimolecular layers?

Explanation

Chemisorption occurs when reactant molecules form covalent or ionic bonds with the surface of adsorbent. Once the layer of reactant molecules has been formed on the surface of adsorbent, no other active site is available for bonding of reactant-adsorbent to form further layers. Physisorption is simply driven by van der Waals. forces of attraction. So, number of layers may be formed one over the other on the surface of the adsorbent