We present the results of high-energy resolution core level photoelectron spectroscopy experiments paralleled by density functional theory calculations to investigate the electronic structure of highly undercoordinated Pt atoms adsorbed on Pt(111) and its correlation with chemical activity. Pt4f(7/2) core level binding energies corresponding to atoms in different configurations are shown to be very sensitive not only to the local atomic coordination number but also to the interatomic bond lengths. Our results are rationalized by introducing an indicator, the effective coordination, which includes both contributions. The calculated energy center of the valence 5d-band density of states, which is a well known depicter of the surface chemical reactivity, shows a noteworthy correlation with the Pt4f(7/2) core level shifts and with the effective coordination.