In this work, we develop a model which allows for an a priori estimation of whether the chemisorption energies of two adsorbates scale with each other. We use the Hammer-Nørskov model, which suggests that the trends a) Electronic mail: jkno@dtu.dk in adsorption energies of transition metals are related to the bonding between the adsorbate valence states and metal d-bands. The hybridisation energy is described using a modified form of the Newns-Anderson model, combined with an effective expression for the orthogonalisation energy. The model demonstrates that the root cause of lack of scaling between two adsorbed species lies in differences of their re-normalised adsorbate valence energies. For instance, O-p states are considerably lower in energy than C-p states. Through this analysis, we identify the conditions under which carbon and oxygen do scale with each other.