The fundamental aspects of N2 and 0 2 adsorption in zeolites have been investigated by density functional calculations on different models. Simple systems where these molecules interact with a positive point charge or with isolated Li+ and Na+ cations have led to a qualitative explanation for the N2/02 separation process. A classical description involving electrostatic and induction energies is adequate to explain the basic reason for a stronger N2 adsorption. At short distances (bonding interaction), the electronic structure of the cation has to be taken into account. The presence of core electrons in large cations limits the stabilizing contribution of the electrostatic and induction terms to the total energy, implying that Li+ is more efficient than Na+ in the adsorption process. The presence of zeolite clusters decreases the binding energies for both N2 and 0 2, but the main trends remain valid. Moreover, due to a larger screening of 0 2 adsorption, it improves the efficiency of Li+ with respect to Na+ for the N2/02 separation.