Physisorption is a general phenomenon and occurs in any solid/fluid or solid/gas system. Chemisorption is characterized by chemical specificity. In physisorption, perturbation of the electronic states of adsorbent and adsorbate is minimal. The adsorption forces include London Forces, dipole-dipole attractions, dipole-induced attraction and "hydrogen bonding." For chemisorption, changes in the electronic states may be detectable by suitable physical means, in other words, chemical bonding. Typical binding energy of physisorption is about 10–300 meV and non-localized. Chemisorption usually forms bonding with energy of 1–10 eV and localized. The elementary step in physisorption from a gas phase does not involve activation energy. Chemisorption often involves an activation energy. For physisorption gas phase molecules, adsorbates, form multilayer adsorption unless physical barriers, such as porosity, interfere. In chemisorption, molecules are adsorbed on the surface by valence bonds and only form monolayer adsorption. A direct transition from physisorption to chemisorption has been observed by attaching a CO molecule to the tip of an atomic force microscope and measuring its interaction with a single iron atom. This effect was observed in the late 1960s for benzene from field emission as reported by Condon and ESR measurements as reported by Moyes and Wells. Another way of looking at this is that chemisorption alters the topology of the electrons in the adsorbate molecule (by the process of chemical reaction) but physisorption does not.
physiSoRptIon is A geNEral pHeNomEnon and OCcuRS in Any SolId/flUiD or soLid/gAs System. chemIsoRptioN Is ChAracteriZed bY cHEmical SpEciFICity. In PhYsiSORpTion, PeRturBATioN of the ElecTronic sTates of adsOrBenT anD adsorbAte Is Minim,al. tHe adsorption forces iNcluDe lOndon fOrces, dipole-diPOle aTtractIONs, dIpole-Induced AttraCtion AnD "hydroGen BonDiNG." foR chEmISOrPtIon, chaNGes In the eLecTrOnIc stATes may be DetecTabLe by suiTable physical meAns, in othEr words ChemiCaL bOnding. tyPiCaL binding ENergY of physIsorptIOn Is aBout 10–300 MEv aNd non-LocalizED. ChemIsOrPTIon usually formS bondinG with enERgy oF 1–10 eV aND lOcAliZEd. tHE eLementAry STep In PHysISoRptIon FRom a gAs PHase dOeS not involve acTiVAtion EnErgy. chEMiSorption OFtEn invOlVeS An activatiON eNergY. fOr phYsisORpTiON Gas phASe MolecuLes, AdSOrBAtes, FOrm multilayEr adsOrptiON UnLESs physIcal BaRriers, Such aS porosity, iNTErFERe. IN chemisoRpTIoN, molecuLes are adsORbed ON the SUrfAce by VAleNCe bONdS and only form monoLayer adSorptIon. A dirEcT transiTIOn fRom phYSiSORptIOn to chEMiSorPTiON has BeEn oBSeRVed bY attachiNG A co moleCUle To the tip oF aN aToMic forcE MicROsCopE And mEAsuring its interacTIOn With A single iRon Atom. thiS EffEct was OBsErVeD in the latE 1960s For benZENE FROM FieLd Emission aS repOrted By COnDON aNd esR meASUrEMENts aS rEpOrTed by Moyes and wells. anoTher waY of looking at thiS Is ThaT cheMISorpTion altErs The topOlOgy oF thE eLEctROns in tHe AdsorbAtE molEcuLe (by tHE prOCEsS of chEmical ReacTiOn) bUt pHYsiSoRptioN does Not.