some inorganic (and problematic organic) compounds such as hydrogen sulfide (H 2 S), ammonia (NH 3 ), formaldehyde (HCOH), radioisotopes iodine-131 ( 131 I) and mercury (Hg). This property is known as chemisorption . [ edit ] Iodine number Many carbons preferentially absorb small molecules. Iodine number is the most fundamental