Electronegativity, described by Linus Pauling described as “The power of an atom in a molecule to attract electrons to itself” (Pauling in The nature of the chemical bond, 3rd edn, Cornell University Press, Ithaca, p 88, 1960), is used to predict bond polarity. There are dozens of methods for empirically quantifying electronegativity including: the original thermochemical technique (Pauling in J Am Chem Soc 54:3570–3582, 1932), numerical averaging of the ionisation potential and electron affinity (Mulliken in J Chem Phys (...) 2:782–784, 1934), effective nuclear charge and covalent radius analysis (Sanderson in J Chem Phys 23:2467, 1955) and the averaged successive ionisation energies of an element’s valence electrons (Martynov and Batsanov in Zhurnal Neorganicheskoi Khimii 5:3171–3175, 1980), etc. Indeed, there are such strong correlations between numerous atomic parameters—physical and chemical—that the term “electronegativity” integrates them into a single dimensionless number between 0.78 and 4.00 that can be used to predict/describe/model much of an element’s physical character and chemical behaviour. The design of the common and popular medium form of the periodic table is in large part determined by four quantum numbers and four associated rules. However, adding electronegativity completes the construction so that it displays the multi-parameter periodic law operating in two dimensions, down the groups and across the periods, with minimal ambiguity. (shrink)

Electronegativity is a quantified, typical chemical concept, which correlates the ability of chemical species to attract electrons during their contact with other species with measurable quantities such as dissociation energies, dipole moments, ionic radii, ionization potentials, electron affinities and spectroscopic data. It is applied to the description and explanation of chemical polarity, reaction mechanisms, other concepts such as acidity and oxidation, the estimation of types of chemical compounds and periodicity. Although this concept is very successful and widely used, and (...) in spite of the fact that it is still subject to scientific investigations, neither a more than intuitive definition nor a generally accepted, logically clear and standardized quantification model has been developed. In the present work, electronegativity is presented and discussed with respect to its main conceptual and operational continuities and discontinuities. We try to analyze the epistemological status of electronegativity, conceived as a typical notion of chemical sciences. Under ‘epistemological status’ we subsume the issues of its reference, its historical persistence, and the relationship between its measurement and quantification. (shrink)

The potential reducibility of chemical entities to their physical bases is a matter of dispute between ontological reductionists on one hand, and emergentists on the other. However, relevant debates typically revolve around the reducibility of so-called ‘higher-level’ chemical entities, such as molecules. Perhaps surprisingly, even committed proponents of emergence for these higher-level chemical entities appear to accept that the ‘lowest-level’ chemical entities – atomic species – are reducible to their physical bases. In particular, the microstructural view of chemical elements, actively (...) developed and defended by emergentists, appears to hold that the explanatory power of nuclear charge justifies being reductionist about atomic species. My first task in this paper is to establish that nuclear charge cannot ultimately provide explanations sufficient to justify a reductionist approach to atomic species, unless we abandon the persuasive intuition that the presence of an element in a substance ought to explain the properties of that substance. The ‘missing piece’ for explaining the properties of substances by way of their elemental constituents is the electronegativity values of participant atoms. But electronegativity is a strikingly disunified concept that appears distinctly unamenable to analysis by way of fundamental physical principles. Through evaluating the uncertain physical identity of electronegativity, as well as its widespread and indispensable epistemic utility in chemical practice, I argue that electronegativity provides compelling grounds to seriously consider emergence for atomic species. (shrink)

Formal charge and oxidation state are two means of estimating the charge of an atom in a molecule. Though these concepts have very different origins—formal charge is derived from the ball-and-hook model of bonding and oxidation state is based on the ionic approximation of molecules—they are used to predict reactivity and other molecular properties through their properties as charges. In this submission, it is shown that formal charge and oxidation state are two extreme descriptions of bonding: formal charge represents zero (...) charge transfer between atoms while oxidation state represents complete charge transfer in each bond. These ‘localised electron approximations’ form an incomplete picture of atomic charge. Electronegativity measures the extent of polarity in real bonds; this concept can be introduced to polarise bonds relative to the ‘equal sharing model’. It is shown that the various electronegativity models are fundamentally related. We chose two models to demonstrate numerically that polar bonds yield charges intermediate between the localised electron approximations: Pauling and Mulliken. It is shown that probabilistic interpretation of the Pauling scale (‘scaled Pauling’ method) and use of asymmetric chemical potential (‘modified Mulliken’ method) lead to atomic charges that closely approximate experimental values using simple ‘back of the envelope’ calculations. It is seen that formal charge, oxidation state, and electronegativity-interpolated charge lie on a continuum and are mathematically related. It is therefore concluded that electronegativity introduces (quantum) delocalisation to the localised (classical) picture of electron bonding. (shrink)

Electronegativity is an important physico-chemical concept to study the chemical structure and reactivity. Although, the conundrum related to measurement of electronegativity still persists. In view of this fact, a simple yet rigorous scale of electronegativity, invoking an inverse relationship with atomic nucleophilicity index, has been proposed for 103 elements of the periodic table. The computed data follows periodicity distinctly satisfying all the sine qua non of a standard scale of electronegativity. Further, electronegativity values display a (...) sound similarity with the standard electronegativity scales validating the suitability of the proposed model. Molecular electronegativities of some polyatomic molecules have also been calculated using the proposed scale of electronegativity.Graphic. (shrink)

During the second half of the nineteenth century, electronegativity has been one of the most relevant chemical concepts to explain the relationships between chemical substances and their possible reactions. Specifically, EN is a property of the substances that allows them to attract external electrons in bonding situations. The problem arises because EN cannot be measured directly. Indeed, the only way to measure it is through different properties that do can be directly measured, for instance enthalpy, ionization energies or electron (...) affinities. What is particularly troubling about this case in quantum chemistry is that the different models used to describe and quantify EN are incompatible but, in a certain sense, equivalent because the same EN scale results in all of them. By analyzing Linus Pauling’s and Robert Mulliken’s models of EN, it will be argued that, if we remain cling to a traditional representative conception of models, we cannot understand the meaning of the information provided by those models. Indeed, if we do not want to adopt an instrumentalist point of view concerning chemical knowledge, we should reconsider by virtue of what a model represents the system, or, in other words, which the factors that determine the representative power of a model are. I will propose a new perspective that incorporates the role of experimental techniques in the very notion of representation; this perspective allows us to understand how supposedly incompatible models of the same target system can be both simultaneously representative. (shrink)

Slater’s method is an integral part of the undergraduate experience. In actuality, Slater’s method is part of an atomic model and not simply a set of rules. Slater’s rules are a simple means for computing the effective nuclear charge experienced by an orbital. These rules are based on the shell-like structure of the Slater atom in which outer shell electrons are incapable of shielding inner electrons. Slater’s model provides a qualitative description of the valence electrons in multi-electron atoms with tremendous (...) ease. The model is useful for explaining atomic properties such as ionisation energy, electron affinity and atomic radius qualitatively. Slater’s rules also correctly reproduce the Madelung rule of filling and the ionisation sequence (4s before 3d); however, these rules are not able to reproduce the anomalous configurations of atoms such as Cr and Cu. It is found that the atomic properties that Slater’s model reproduces are all related to the exponential decay factor of the Slater orbital. We find—from estimating the polarity of a diatomic molecule using a simple model—that molecular polarity is related to the difference in the exponential decay factors of the valence orbitals of the two atoms, implying that the decay factor acts as the electronegativity of the atom. (shrink)

Recently, philosophers have come forth with approaches to chemistry based on its actual practice, imparting to it a proper aim and character of its own. These approaches add to the currently growing movement of pluralist philosophies of science. We draw on recent pluralist accounts from chemistry and analyse three notions from modern chemical practice and theory in terms of these accounts, in order to complement the so far more general pluralist approaches with specific evidence. Our survey reveals that the concept (...) of element indicates conceptual pluralisms in chemistry. that electronegativity illustrates methodological pluralism in determining one and the same concept, and that acidity is an instance of plurality hidden behind ‘one notion’. (shrink)

In modern German ‘Anschauung’ is translated as intuition. But in Kant’s technical philosophical context, it means an intuition derived from previous visualizations of physical processes in the world of perceptions. The nineteenth century chemists’ predilection for Kantian Anschauung led them to develop an intuitive representation of what exists beyond the bounds of the senses. Molecular structure is one of the illuminating outcomes. (Ochiai 2021, pp. 1–51) This mental habit seems to be dominant among chemists even in the twentieth century, as (...) is illustrated by the electronic theory of organic chemistry and the frontier orbital theory as well. The former assumes that (1) bonds are paired electrons shared by bonded atoms—in fact, electrons in molecules are not localized in bonds; (2) the difference of electronegativities between bonded atoms causes electron drifts—expressed by the curly arrow—that result in bond formation or bond cleavage. The latter focuses on the orbitals that make the greatest contribution to the energy of a system undergoing electron delocalization, while the LCAO method says, as is suggested by the word Linear Combination of Atomic Orbitals, molecular orbitals should be constructed from all of the atomic orbitals that have the appropriate symmetry. In other words, every molecular orbital contributes to some extent to the electronic state of a molecule. The curly arrow in the electronic theory and the orbital lobe in the frontier orbital theory illustrate an intuitive character of these theories. Although both theories rely on such simple and qualitative models rather than mathematically rigid quantum mechanical calculations, they are successful in explaining, predicting, and designing chemical reactions. What makes these prima facie intuitive theories so successful? In this study we address this problem from a historical and philosophical as well as scientific point of view. The key to solve this problem is that they are concerned with only bond formation or bond cleavage, in which the localized-bond principle holds. (shrink)

Examples are given of applications by Pauling, Mulliken, Marcus and G.E.Kimball of the three Pythagorian means to formulate the scales of electronegativity of the elements, to the calculations of rate constants of electron transfer cross-reactions, to the calculation of the observed rate constant as function of activation and diffusion rate constants in the case of mixed reaction-diffusion rates and to the calculation of the effective diffusion coefficient in solution of a salt AB as a whole from the diffusion coefficients (...) of the ions in which it dissociates. (shrink)

We review the early works which were precursors of the Conceptual Density Functional Theory. Starting from Thomas–Fermi approximation and from the exact formulation of Density Functional Theory by Hohenberg and Kohn’s theorem, we will introduce electronegativity and the theory of hard and soft acids and bases. We will also present a general introduction to the Fukui functions, and their relation with nucleophilicity and electrophilicity, with an emphasis towards the importance of these concepts for chemical reactivity.