In physical adsorption, the molecules of adsorbate are attached to the bottom of the adsorbent by attraction of physical forces or wander wall forces.
This attraction is weak. In chemical adsorption, the atoms of the adsorbate are joined to the atoms at the bottom of the adsorbent by chemical bonds.
All the valences of the atoms inside the adsorbent are complete. The atoms at the bottom of the adsorbent do not have all their valences.
These are called free valences. Due to the free valence at the bottom of the adsorbent, a chemical bond is formed between the adsorbent and the adsorbate atoms.