The new model describes adsorption isotherms with a two-parameter function, The parameters are easy and precise to calculate from the excess surface work of the adsorbed layer, The excess surface work is defined as the product of adsorbed amount and the change of the chemical potential, The differential form yields a minimum at the first layer. It is possible to linearize the corresponding integral form, The physical meanings of the two parameters are the monolayer capacity and the chemical potential, which is necessary for adsorption, The new model allows an energetic classification of adsorption phenomena of surface films in the energetic range from weak physisorption to chemical adsorption.