Ralph Gottfrid Pearson (January 12, 1919 – October 12, 2022) was an American physical inorganic chemist best known for the development of the concept of

hard and soft acids and bases HSAB concept is a jargon for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemic ...

(HSAB). He received his Ph.D. in physical chemistry in 1943 from

Northwestern University Northwestern University is a private research university in Evanston, Illinois. Founded in 1851, Northwestern is the oldest chartered university in Illinois and is ranked among the most prestigious academic institutions in the world. Chart ...

, and taught chemistry at Northwestern faculty from 1946 until 1976, when he moved to

University of California at Santa Barbara The University of California, Santa Barbara (UC Santa Barbara or UCSB) is a public land-grant research university in Santa Barbara, California with 23,196 undergraduates and 2,983 graduate students enrolled in 2021–2022. It is part of the Un ...

(UCSB). He retired in 1989 but remained active in research in theoretical inorganic chemistry until his death. In 1963 he proposed the qualitative theory of

(HSAB) in an attempt to unify the theories of reactivity in inorganic and organic chemistry. In this theory 'Hard' applies to species that are small, have high charge states, and are weakly polarizable. 'Soft' applies to species that are large, have low charge states and are strongly polarizable. Acids and bases interact, and the most stable interactions are hard–hard and soft–soft. In 1958 Pearson and Fred Basolo, his colleague at Northwestern wrote the influential monograph "Mechanisms of Inorganic Reactions", which integrated concepts from

ligand field theory Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine vale ...

and physical organic chemistry and signaled a shift from descriptive

coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ''ligands'' or complexing ...

to a more quantitative science. With another Northwestern colleague, Arthur Atwater Frost, Pearson wrote in 1961 another classic text, ''Kinetics and Mechanism: A Study of Homogeneous Chemical Reactions'' (). A subsequent edition was with John W. Moore as co-author (). In 1983 in collaboration with Robert Parr, he refined the HSAB theory into a quantitative method by calculating values of “absolute hardness” using

density functional theory Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-bo ...

, an approximate method in molecular quantum mechanics. This concept of "absolute hardness" was later connected with the concept of (absolute)

electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

. Pearson died on October 12, 2022, at the age of 103.

Honours

American Chemical Society The American Chemical Society (ACS) is a scientific society based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has more than 155,000 members at all ...

Award for Distinguished Service to Inorganic Chemistry 1970 * Member of

National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the Nat ...

1974

References

External links

Illinois, Cook County Birth Certificates, 1878-1922
{{DEFAULTSORT:Pearson, Ralph 1919 births 2022 deaths 21st-century American chemists Inorganic chemists Members of the United States National Academy of Sciences University of California, Santa Barbara faculty American centenarians Men centenarians Northwestern University alumni Northwestern University faculty Scientists from Chicago