@ adyasha
actually adsorption is a surface phenomenon , and it means the gas particle will remain adsorbed , that means it will remain stick to the surface of solid , beacuse the solid surface will exert some kind of force for that .
and for that when the gas particle will get trapped in the solid surface , obviously their motion will get restricted after that .,
and gas particle , as they have very low force of attraction , they will going to remain attched in that surface only , they cannot come outside .
HOPE THE REASON ARE CLEARS TO YOU
ALL THE BEST ..