Ralph G. Pearson (1963) brought forth a concept by which chemical reactivities, selectivities, and stabilities of chemical compounds may be readily rationalized. Chemical species, including atoms, molecules, ions,and free radicals are categorized as “hard” and “soft" Lewis acids or bases. The “hard” species in general have small radius, high nuclear charge, and low polarizability, whereas “soft” ones possess the opposite characteristics. Over time, many parts of chemistry have been identified that can be rationalized by using the HSAB principle. Including coordination compounds, charge-transfer complexes, hydrogen bonding, free-radical complexes, solvent-solute interactions, solid-state compounds,. Indeed, most inorganic and organic molecules can be thought of as acid-base complexes, Pearson notes one of the pitfalls of the HSAB theory is that the adjectives “hard” and “soft“ don't mean the same as strong and weak. Which makes it difficult in general to quantify the strength of hard and soft acids and bases.The impossibility of establishing a universal scale of Lewis basicity (acidity) is the reason for the existence of many scales of acitidy (basicity). Moreover, although these scales are generally not interrelated, there exist family-dependent relationships that permit ranking, in a rather general order, of bases belonging to a given chemical family, for example, the family of oxygen bases. Therefore, the skepticism about the quantitative usefulness of the Lewis concept of acids and bases still exists.