Chemists develop new scale for electronegativity

By: Staff,  A&S Communications
January 21, 2019

Electronegativity is one of the most well-known models for explaining why chemical reactions occur. Used daily by chemists and materials researchers all over the world, the theory of electronegativity is used to describe how strongly different atoms attract electrons. In a new paper, researchers have redefined the concept with a more comprehensive electronegativity scale.

“A table of electronegativities is as handy to have as the Periodic Table – it tells you at glance how electrons shift when two or three elements come together in a compound. Our new electronegativity scale comes equally from theory and experiment and it covers just about all the elements with which chemists can play,” said Nobel Laureate Roald Hoffmann, the Frank H.T. Rhodes Professor of Humane Letters Emeritus in the College of Arts and Sciences.

Electronegativity Seen as the Ground-State Average Valence Electron Binding Energy, published in the Journal of the American Chemical Society, was co-authored by Hoffman, Martin Rahm, Chalmers University of Technology and Tao Zeng, Carleton University in Canada.

By using electronegativity scales, the approximate charge distribution in different molecules and materials can be predicted without needing to resort to complex quantum mechanical calculations or spectroscopic studies. This is vital for understanding all kinds of materials, as well as for designing new ones. 

"We derived these values by combining experimental photoionization data with quantum mechanical calculations,” said Rahm. “By and large, most elements relate 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 places in the order of electronegativity. Additionally, for some elements this is the first time their electronegativity has been calculated."

The new scale encompasses 96 elements, a marked increase from previous versions. The scale now runs from the first element, hydrogen, to the ninety-sixth, curium. Compared to earlier scales, oxygen and chromium have both been moved in the ranking, relative to elements closest to them in the periodic table.

One motivation for the researchers to develop the new scale was that, although several different definitions of the concept exist, each is only able to cover parts of the periodic table. An additional challenge for chemists is how to explain why electronegativity is sometimes unable to predict chemical reactivity or the polarity of chemical bonds.

The new scale’s definition fits into a wider framework that can help explain what happens when chemical reactions are not controlled by electronegativity. In these reactions, which can be hard to understand using conventional chemical models, complex interactions between electrons are at work. What ultimately determines the outcomes of most chemical reactions is the change in total energy. In the new paper, the researchers offer an equation where the total energy of an atom can be described as the sum of two values. One is electronegativity, and the second is the average electron interaction. The magnitude and character of these values as they change over a reaction reveals the relative importance of electronegativity in influencing the chemical process.

There are endless ways to combine the atoms in the periodic table to create new materials. Electronegativity provides a first important indicator of what can be expected from these combinations.