The oxidation state is a fundamental chemical concept commonly employed to rationalize, classify, and predict the chemical reactivity of a variety of compounds. Understanding and defining the elemental oxidation state of solid materials at the atomic level becomes increasingly complex as their physical dimensions are reduced from tens of nanometers─where properties are still dominated by bulk or outer atomic crystal plane characteristics, to the subnanometric limit. In this work, we highlight the significant limitations in determining even a basic quantity, such as the oxidation state, when oxidized low-nuclearity mass-selected clusters are investigated by means of X-ray photoelectron spectroscopy (XPS), widely recognized as the elective approach to resolve different oxidations states. The lack of crystalline order in these nanoclusters, unlike that in periodic bulk systems and in solid surfaces, leads to a broad distribution of measured core levels, as shown in the case study of W nanoclusters. These cannot be unambiguously assigned to a valence state based solely on the knowledge of bulk matter behavior but need close comparison with specific theoretical modeling. Our results emphasize the substantial challenges inherent in understanding the unique properties of nanoscale materials, particularly in making a rigorous and quantitative determination of a fundamental property that takes a relevant role in many chemical processes, and represent crucial knowledge for advancing technologies that rely on the miniaturization of matter in various processes.
