Electronegativity is a fundamental concept in chemistry. Despite its importance, the experimental determination has been limited only to ensemble-averaged techniques. Here, we report a methodology to evaluate the electronegativity of individual surface atoms by atomic force microscopy. By measuring bond energies on the surface atoms using different tips, we find characteristic linear relations between the bond energies of different chemical species. We show that the linear relation can be rationalized by Pauling's equation for polar covalent bonds. This opens the possibility to characterize the electronegativity of individual surface atoms. Moreover, we demonstrate that the method is sensitive to variation of the electronegativity of given atomic species on a surface due to different chemical environments. Our findings open up ways of analysing surface chemical reactivity at the atomic scale.

Department of Advanced Materials Science Graduate School of Frontier Sciences University of Tokyo 5 1 5 Kashiwanoha Kashiwa Chiba 277 8561 Japan

Department of Thin Films and Nanostructures Institute of Physics Czech Academy of Sciences Cukrovarnická 10 112 Prague 16200 Czech Republic

Division of Electrical Electronic and Information Engineering Graduate School of Engineering Osaka University 2 1 Yamada Oka Suita Osaka 565 0871 Japan

Regional Centre of Advanced Technologies and Materials Department of Physical Chemistry Palacky University Šlechtitelů 27 78371 Olomouc Czech Republic

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