Why physical adsorption is multi layered while chemical adsorption is mono layered?

Dear student,
Physical adsorption involves weak van der Waal's forces, so any number of layers may be formed one over the other on the surface of the adsorbent. Chemical adsorption takes place as a result of chemical reaction between adsorbate and adsorbent hence when the total surface is covered with one layer no further reaction is possible.
Regards