Fyzikální ústav Akademie věd ČR

Scanning microscopes can determine the electronegativity of atoms

A new insight into the characterization of chemical properties of the elements has been contributed by a method of Czech and Japanese researchers, published in the prestigious journal Nature Communications. State-of-the-art scanning-probe microscopes already enable scientists to resolve individual atoms on surfaces, but thanks to the new method, they can also measure the ability of these atoms to attract electrons, i.e. their electronegativity. The new method also makes it possible to measure the variability of electronegativity of individual atoms depending on their chemical environment. This opens the way to deeper understanding of the nature of chemical bond and chemical processes on the atomic level. Such new findings could allow control of chemical reactions in catalysis or biochemistry.

The electronegativity determines, among other things, the ability of an atom to interact with its environment and create chemical bonds. Until recently, scientists have been able to quantify it only using techniques that have worked with a large ensemble of atoms. The possibility of determining the electronegativity of a given atom depending on the chemical environment has not been possible so far. This situation has changed due to the collaboration of scientists from the Institute of Physics of the Czech Academy of Sciences, the Palacký University in Olomouc and the Universities of Tokyo and Osaka.

Schematic representation of the interaction between the tip of the atomic force microscope and surface atoms. Different energy of interactions E of a probe with individual surface atoms allows one to obtain information on the electronegativity of individual atoms, which is given not only by their own chemical nature but also depends on their chemical environment. The figure depicts an electron density around atoms. The differences of the electron density among the atoms signify their different electronegativities.

“The new method enables us, by means of atomic force microscopy, not only to determine the electronegativity of an atom on the surface of a solid, but we are also able to determine its dependence on the chemical environment of the measured atom. That was not possible before. The method is based on experimental measurements of binding energies of individual surface atoms supported by theoretical calculations. The detailed analyses showed how the chemical environment affects the electronegativity of the atom. We can use this knowledge for targeted management of chemical reactions." said Pavel Jelínek, who works at the Institute of Physics of the Academy of Sciences of the Czech Republic and the Regional Center of Advanced Technologies and Materials (RCPTM) at Palacký University in Olomouc. This work extends the team's own research published in 2007 in the Nature magazine. The new method, however, overcomes the limitations of the original approach of chemical identification of atoms by making it possible to identify chemical elements with different electronegativity.

"We have shown that the current values of electronegativity of chemical elements are only valid for isolated atoms. Our method allows us to determine its changes based on the chemical environment of the atom. This gives us a new, complex view of electronegativity and therefore we have to look a little differently at characters of bonds in the chemical compounds and on the chemical reactivity itself." added Pavel Jelínek. The most electronegative elements in the periodic table are halogens headed by fluorine, the least electronegative are alkali metals.

The researchers have also demonstrated a characteristic linear relationship between the binding energies of surface atoms of different elements. Thus they demonstrated experimentally the validity of the equation proposed by Nobel Prize winner Linus Pauling for the polar covalent bond in the 1930s.

J. Onoda, M. Ondáček, P. Jelínek, Y. Sugimoto, Electronegativity determination of individual surface atoms by atomic force microscopy, Nature Communications 8, 15155 (2017).

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