From Electronegativity towards Reactivity—Searching for a Measure of Atomic Reactivity

Abstract

Pauling introduced the concept of electronegativity of an atom which has played an important role in understanding the polarity and ionic character of bonds between atoms. We set out to define a related concept of atomic reactivity in such a way that it can be quantified and used to predict the stability of covalent bonds in molecules. Guided by the early definition of electronegativity by Mulliken in terms of first ionization energies and Pauling in terms of bond energies, we propose corresponding definitions of atomic reactivity. The main goal of clearly distinguishing the inert gas atoms as nonreactive is fulfilled by three different proposed measures of atomic reactivity. The measure likely to be found most useful is based on the bond energies in atomic hydrides, which are related to atomic reactivities by a geometric average. The origin of the atomic reactivity is found in the symmetry of the atomic environment and related conservation laws which are also the origin of the shell structure of atoms and the periodic table. The reactive atoms are characterized by degenerate or nearly degenerate (several states of the same or nearly the same energy) ground states, while the inert atoms have nondegenerate ground states and no near-degeneracies. We show how to extend the use of the Aufbau model of atomic structure to qualitatively describe atomic reactivity in terms of ground state degeneracy. The symmetry and related conservation laws of atomic electron structures produce a strain (energy increase) in the structure, which we estimate by use of the Thomas-Fermi form of DFT implemented approximately with and without the symmetry and conservation constraints. This simplified and approximate analysis indicates that the total strain energy of an atom correlates strongly with the corresponding atomic reactivity measures but antibonding mechanisms prevent full conversion of strain relaxation to bonding.

Energy Electronegativity and Chemical Bonding

“…the notions of metallicity, acidity, reactivity, etc. Sixty years have passed since the stormy discussions and condemnations of the EN concept, but its development and application in various fields of science, such as structural chemistry, materials science, molecular spectroscopy, and various fields of physical and inorganic chemistry [ 117 , 118 , 119 , 120 , 121 ] does not stop; it was even suggested to use EN as the third coordinate in the Periodic Table [ 122 ].…”

Classification of substances by health hazard using deep neural networks and molecular electron densities

“…To map physical properties onto the feature space, a local property map of electronegativity was used as a secondary label, as follows. Since high (local) electronegativity is correlated to high (local) reactivity [52], we derive a ternary reactivity mask from the electronegativity cube files for regions of high, medium and low local reactivity. To categorize reactivity, voxels with electronegativity values above the 90th percentile were labeled as class 2, signifying high reactive sites.…”

“…All other voxels are marked as 0 and shown as green higher than the 90th percentile were marked as class 2, i.e., high strength electronegative sites, while those less than the 10th percentile were marked as class 1, i.e., low strength electronegative sites, and the remaining voxels were denoted as belonging to class 0, i.e., medium strength electronegative site. These are commonly considered as active sites where reactions would take place [51][52][53][54][55]. Thus, together with electron density distributions they can be used for the classification of compounds into chemical substances that are allowed, i.e., are not hazardous and those that are health hazards and hence prohibited for the two use-cases.…”