Linus Pauling, ca. early 1930s.
Exciting news from the laboratories of Oregon State University: a group of researchers here have developed a method that simplifies the scientific understanding of electronegativity, a concept introduced and greatly advanced by Linus Pauling in the 1930s with his “electronegativity scale.” We’ve written about Pauling’s electronegativity work before and since we’re in an interviewing mood lately, we thought we would catch up with the authors of this new breakthrough to find out what it’s all about.
Below are the fruits of our conversation with OSU’s Ram Ravichandran and Brian Pelatt, both doctoral candidates in electrical and computer engineering and co-authors of this important new paper, “Atomic Solid State Energy Scale.” (J. Am. Chem. Soc., 2011, 133 (42): 16852-16860.)
Pauling Blog: In layman’s terms, what is electronegativity?
Brian Pelatt: Electronegativity as defined by Pauling is “the power of an atom in a molecule to attract electrons to itself.” When two atoms come together to form a molecule, the electronic charge will distribute itself so that one of them will be positively charged and the other negatively charged. Electronegativity is a way to explain that charge redistribution and quantify which atoms will more likely become negatively charged.
In the solid state energy framework, the electronegativity would be a measure of how good an atom is at taking, or “stealing”, electrons (negative charge) from other atoms. As an example, fluorine is the most electronegative element and has the largest solid state energy, so it will almost always take negative charge from another element when they bond.
Electronegativity calculations by Linus Pauling, ca. 1930s.
What was Linus Pauling’s contribution to our understanding of electronegativity?
Ram Ravichandran: The concept of electronegativity was first proposed by Pauling in 1932. In an effort to explain chemical bonding, Pauling looked at chemical bond energy data derived from thermochemical measurements. He noticed that the bond energy of dissimilar atoms was greater than the covalent bond of similar atoms. This difference, what he called the ionic character of the bond, gave rise to the electronegativity scale. However, his scale was arbitrary. He assigned a value of 4.0 to fluorine and then proceeded to use that value to calculate his electronegativity scale.
What has your group done now to further the concept of electronegativity?
Pelatt: Our group has furthered the concept of electronegativity by simplifying it in the Solid State Energy (SSE) framework. Now, instead of an element having an arbitrary number assigned to it as the electronegativity, it has a solid state energy that is based on experimental measurements and easy to understand. As can be seen from the electronegativity definition given earlier, electronegativity is a difficult concept both for students and teachers; this work simplifies the concept significantly. This approach can be used to simplify difficult chemical concepts such as the Hard-Soft Acid-Base Principle, chemical hardness, covalent vs. ionic bonding, and acidic/basic oxides.
The SSE approach is also a way to predict the band gap of a material, which is a fundamental property of materials and determines the behavior of the material, whether it behaves as a conductor, semiconductor, or insulator. Another advantage of the SSE approach is that it is based on solid state measurements, rather than gas phase measurements. Solid state electronics is the foundation of modern devices, having a scale that is tailored to the solid state is a huge advantage of the SSE concept.
How long have you been working on the problem?
Pelatt: We have been working on this for about a year, with most of the time collecting and interpreting data.
What methods did your group use to arrive at your conclusions?
Ravichandran: At the beginning, we did not anticipate working on electronegativity. We began looking at doping trends and energy band offsets of compounds to see if we could come up with an ability to predict properties of new materials.
A concept that connects chemistry and device engineering is the position of energy bands. In chemistry, these bands are known as HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) which translate to the valence band and conduction band in device engineering. The separation between the bands is the energy band gap. The position of these bands is a measurable quantity known as ionization potential, IP (for HOMO, or valence band) and Electron Affinity, EA (for LUMO, or conduction band).
We started tabulating the values for EA and IP for about 69 compounds, and when plotted against the band gap, we noticed a curious trend. The values were centered around 4.5 eV, which corresponds to the standard hydrogen potential commonly used in electrochemistry and is also the bond strength of a hydrogen atom. This led us to conclude that hydrogen could serve as a universal energy reference position.
In chemistry, for a binary A-B compound, the HOMO is predominantly anion derived (mostly B), while the LUMO is cation derived (mostly A). Since the EA is related to the LUMO, by collecting and averaging the EA values for a cation such as aluminum, we were able to come up with a new “solid state energy” (SSE) value for aluminum. Similarly, by collecting and averaging IP values for nitride materials, we were able to calculate an SSE value for nitrogen.
When we then organized these SSE values for 40 elements in an increasing order along with the universal energy reference, common chemistry concepts such as oxide clarification, electronegativity, chemical hardness and ionicity became easier to interpret.
Where do you go from here?
Ravichandran and Pelatt: That is a very interesting question. Within our group, we are already utilizing this concept to be able to predict properties of new compounds. Dr. Michael Lerner and Dr. Richard Nafshun in the Chemistry department are both excited about incorporating this concept in graduate and undergraduate chemistry courses. We believe that the SSE concept has a wide reaching audience, from those trying to understand chemistry concepts to using this scale to investigate properties of new materials with applications in battery technology, water splitting and solar cells, amongst others.
Another exciting path is to see where the scientific community at large takes the SSE concept. The SSE values can be refined and this should spark a lot of conversation about measurement techniques. Doing so would provide a lot of subtle information about chemical bonding that is not readily apparent. The SSE scale helps to quickly estimate important properties of new materials, enabling rapid material development for applications. As a result, the most exciting aspect of SSE is how the community is going to use this scale in the search for materials with novel applications.
For more on this breakthrough in our understanding of electronegativity, see this press release issued by OSU.
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