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  • Alberto Barosa Nuñez

Electronegativity: an intrinsic or extrinsic property?

A post by Alberto Barosa Nuñez

@albertonuno7


Many of the properties and phenomena within chemistry have aspects that can be of great interest to the philosophy of science. Among them is electronegativity, the subject of this post, which is of great importance as well as considerable ambiguity. This ambiguity results from its different approaches and makes us question its ontological nature: is it an intrinsic or extrinsic property?


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What is electronegativity?

First of all, what is electronegativity (EN)? This concept can be defined as the capacity of chemical species to attract electrons towards itself in a chemical bond (Ruthenberg and Martínez González, 2017). Chemists consider it is a key factor to explain the mechanisms of most reactions.

As can be seen in the figure below, carbon is more electronegative than Zinc (Zn). On these grounds, carbon (1) is said to adopt a negative charge that in turn accounts for breaking the bond in the other compound between nitrogen (N) and carbon (2)—which has a positive charge because nitrogen is more electronegative.





The two approaches

Despite the great relevance of EN in explaining these mechanisms, there are certain issues that reveal its ambiguity (Ruthenberg and Martínez González, 2017), as the fact that counts with 20 models aiming to explain this concept. The focus of the subsequent discussion will be on two main approaches (Accorinti and Labarca, 2020):

1) The thermo-chemical approach. The  most known model is the one coined by Pauling. The first step of this model consists in calculating the difference between the bond dissociation energy of AB (which is said to be the actual covalent bond) and the bond dissociation energy of both A2 and B2 (which is said to be the purely covalent bond):


AE = E(AB) = E (AB) exp - ½ [Eaa + Ebb]


The second step consists in obtaining the values of EN of both A and B with the following formula using the value calculated in the previous step:


χA - χB = (ΔE/2)/0.208


But this description can only establish relative electronegativity, so it is necessary to establish previously one element as the reference, which Pauling set to be hydrogen.


2) The spectroscopic approach. This approach provides two models, of which Mulliken's is the better known. Mulliken used two atomic properties, the ionization potential (I)—the measure of the energy required to remove an electron from an atom in its ground state— and the electron affinity (A) —the energy change that occurs when an atom gains an electron to form a negatively charged ion:

XA = ½ (IA + AA)


To put it briefly, EN is conceived by Pauling in terms of the difference between the energy of the actual covalent bond (obtained thermodynamically) and the energy of the purely covalent bond (computed in a theoretical way), for any two substances. Whereas EN is conceived by Mulliken in terms of the balance between the tendency to lose (I) and to gain (A) electrons within an atom.


Intrinsic or extrinsic?

After explaining the difference between the two models on a scientific level, it is time to explore the ontological incompatibility: while Pauling conceives EN as a property belonging to the molecule as a whole instead of to the atoms, Mulliken conceives it as a property belonging to the atoms in virtue of its characteristics. As Martín Labarca (2021) highlights, that means that EN is an extrinsic property from the thermo-chemical approach (Pauling), and an intrinsic property from the spectroscopic one (Mulliken).


But what do we mean with extrinsic and intrinsic? David Lewis is the most quoted author to answer this question (1983, 197), and the following table illustrates the idea that most of the texts share about them:


Intrinsic properties

Extrinsic properties

Depends

only on its bearer

as well on the things that surround its bearer

Examples

Mass, hardness, etc.

Weight, volume

The mass of an object is the consequence of aspects such as the number of particles, constituent of the object itself and not a direct consequence of its bearer surroundings. On the other hand, weight depends in turn on the strength of the gravitational field, that is why this property, unlike mass, is an extrinsic one.


This incompatibility prompts a philosophically significant issue, for it is part of several discussions  on metaphysics and ethics (Marshall and Weatherson, 2002). For example, environmental ethics is based on an axiology that sets which elements in nature (conscious beings, living things, intelligent beings, etc) might have intrinsic value to be preserved, i.e, which of these beings or things should be preserved considering their value per se and not only the value they may have for humans.


It is important also to know which kind of events/phenomenon we are addressing since it helps us to know better the elements that originate them. Depression or stress could be regarded as good examples of this: is depression intrinsic or extrinsic to the people who suffer from it? What are the intrinsic and extrinsic aspects that cause stress to a person?


We can also note how our society has historically made us believe that some of our capacities or tendencies (gender, sex, willingness to earn money, etc.) are intrinsics to the "human nature" —despite the great difficulty of its definition— and not an extrinsic one that have been shaped by our socioeconomic contexts. In fact, the research into what kind of traits are intrinsic to human nature has been an unavoidable discussion among authors who have dealt with this question (Hull, 1986).


This topic is usually coupled with the distinction between a ‘genuine change’ and a ‘mere Cambridge’; the former is a change in intrinsic properties provoking substantial modifications in the rest of its bearers properties while the later is a change in extrinsic ones and doesn’t lead to significant developments. When Bruce Wayne’s parents died, he instantly became an orphan, but that didn’t constitute a real change in himself as a human being (despite the great transformation it had in the way he functions in society), whereas a change in one of the functional groups of a molecule does lead for sure to a change in its physical properties.





Conclusion

Thanks to these concepts with a large bibliography in the field of metaphysics, it has been possible to show the theoretical particularity of this property within the field of science. Even though both of the EN models have been proven to be empirically effective, they have very different conceptions from an ontological point of view and because of this there are still discussions about the best way to unify them. The case analyzed in this post is a clear example of the philosophical complexity that the plurality of models in science can reach.



References


Hull, David. “On human nature”. PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association, no. 2, 1986, pp. 3–13


 Ruthenberg, Klaus,  and Juan Camilo Martínez González. “Electronegativity and its multiple faces: persistence and measurement.” Foundations of Chemistry, vol. 19, 2017, pp. 61-75.


Labarca, Martín. “Por un diálogo entre la filosofía de la química y la enseñanza de la química: el caso de la electronegatividad.” Educació Química EduQ, no. 29, 2021, pp. 48-53.


Labarca, Martín, and Hernán Accorinti. “Commentary on the Models of Electronegativity.” Journal of Chemical Education, no. 97, 2020, pp. 3473-3477.


Lewis, David. “Extrinsic properties.” Philosophical Studies, no. 44, 1983, pp. 197-200.


Marshall, Dan, and Brian Weatherson. “Intrinsic vs. Extrinsic Properties (Stanford Encyclopedia of Philosophy).” Stanford Encyclopedia of Philosophy, January 2002, https://plato.stanford.edu/entries/intrinsic-extrinsic/#PhilImpo.


Alberto Barosa Nuñez

@albertonuno7


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