Only the surface atoms in an adsorbent ,play an active role in adsorption.These atoms posses some residual forces likes van der waal's forces and chemical forces. ltbRgt In the process of adsorption ,weak adsorbate is constituted by strong adsorbate. Activated charcoal used in gas mask is already exposed to the atmospheric air, so the gases and water vapours in the air are adsorbed on its surface. When the mask is exposed to chlorine atmosphric, the gases are displaced by chlorine. Porous and finely powered solids ,e.g, charcoal and Fuller's earth adsorb more as compared to the hard non-porous material.It is due to this property that the powered charcoal is used in gas masks. In general, easily liquefiable gases likesCO_(2) ,NH_(3) ,Cl_(2) and SO_(2) ,etc. are adsorbed to a greater extent than the elemental gases , e.g, H_(2), N_(2) ,O_(2), He, etc. Which of the following gas molecules has maximum value of enthalpy of physisorption in a gas mask ?