By Prabhat Ranjan and Tanmoy Chakraborty
Invoking the atomic radius as size descriptor, we can easily predict and model so many physico chemical properties of atoms and molecules. Since it has not been possible to isolate an atom and to determine its size, the atomic size is not an observable. The theoretical paradigm to study the electronic structure and properties of atoms, molecules and crystalline bodies is quantum mechanics or wave mechanics. But, radius is not a quantum mechanical observable and cannot be evaluated easily. As we know that atomic radius is periodic in nature, we can compute the absolute size of atom invoking its periodic correlation with other periodic properties. Recently Chakraborty et al. published a new set of theoretical atomic radii on basis of other periodic properties namely effective nuclear charge and ionization energy. In this venture , we have used that particular atomic size to calculate another important periodic parameter namely global hardness (η). Using the inverse relationship between atomic size and global hardness, we have computed a set of atomic hardness for 103 elements of the periodic table. The express periodicity of periods and groups exhibited by the computed atomic hardness and the manifest relativistic effect of the lanthanoids and actinoids, etc. speak volumes for the efficacy of the present method. We have also applied our computed atomic hardness to evaluate another important periodic parameter, electrophilicity index (ω).The absolute electronegativity value evaluated by Chakraborty et al. is the necessary input to compute the electrophilicity index .A comparative study has been done with our computed electrophilicity index values with other reported results. A nice correlation supports the validity of this work.