New Scale for Electronegativity

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Electronegativity is one of the most well-known and used models for explaining why chemical reactions occur. Now, electronegativity is redefined in a new, more comprehensive scale, published in the Journal of the American Chemical Society. Behind the study is Martin Rahm, assistant professor in Physical Chemistry at Chalmers along with one Nobel laureate.

​The theory of electronegativity forms an important basis for understanding why the elements react with each other to form different types of materials with different properties. It is a central concept used daily by chemists and material researchers all over the world. The concept itself originates from the Swedish chemist Jöns Jacob Berzelius’ research in the 19th century, and is commonly taught already at high school level.

Electronegativity describes how strongly different atoms attract electrons. By using electronegativity scales one can predict the approximate charge distribution in different molecules and materials, without quantum mechanical calculations or spectroscopic studies. In this way, electronegativity offers clues to how atoms and molecules will react when assembled. This is very important for understanding all kinds of materials and for designing new ones.

Martin Rahm, assistant professor in Physical Chemistry at Chalmers together with his colleagues Toby Zeng at Carlton University in Canada and Roald Hoffmann, Nobel laureate in Chemistry 1981 working at Cornell University in the United States, has now developed a whole new electronegativity scale, which they have recently published in the Journal of the American Chemical Society. The new scale has been devised by combining experimental photoionization data for atoms and quantum mechanical calculations for those atoms where experiments are missing.

One motivation for the researchers to develop the new scale was that, although there are already several different definitions of the concept, these have only been applied to cover parts of the periodic table. An additional challenge for chemists is how to explain what it means when electronegativity sometimes fails to predict chemical reactivity or polarity of chemical bonds.

“The old and useful concept now has a new definition. The new definition is the average binding energy of the outermost and weakest bound electrons, the so-called valence electrons. These values have been computed by combining experimental data with quantum mechanical calculations. By and large, most elements are relating to each other in the same way as in earlier scales, but the new definition has also led to some interesting changes where atoms have switched place in the ordering of electronegativity.  Some elements have also had their electronegativity calculated for the first time.” says Martin Rahm.

For example, oxygen and chromium have both been moved in the ranking relative to elements closest to them in the periodic table, compared to in earlier scales. The new scale comprises 96 elements, which is a marked increase compared with several previous scales. In this way, electronegativity is available from the first atom, hydrogen or H, to the ninety-sixth, curium or Cm.

An additional advantage of the new definition of electronegativity is that it is part of a framework that can help explain what it means when chemical reactions are not controlled by electronegativity. In such reactions, which can be at odds with conventional chemical rationales, complex interactions between electrons are instead typically governing. What ultimately determines the outcomes of most chemical reactions is changes in the total energy. In their work the authors offer an equation where the total energy of an atom can be described as the sum of two terms, where one term is the electronegativity, and the second describes the average electron interaction. The magnitude and sign of these terms as they change over a reaction reveals the relative importance of electronegativity in governing chemistry.

"This scale is extensive, and I think and hope it will affect research in chemistry and material science. Electronegativity is routinely used in chemical research and with our new scale a number of complicated quantum mechanical calculations can be avoided. The new definition of electronegativity can also be applied for analysing electronic structures calculated quantum mechanically, by making such results more comprehensible." says Martin Rahm.

There are endless ways to combine the atoms in the periodic table and for creating new materials. Electronegativity provides a first important insight into what can be expected from these combinations. Development of numerous novel chemical reactions and materials may speed up due to the new scale. This is because the new definition allows for chemical intuition and understanding that, in turn, can guide both experiments and time-consuming quantum mechanical calculations.