Kasimierz Fajans or Kasimir Fajans (27 May 1887 - 18 May 1975), was a Polish-American chemist who did valuable work on chemical bonding and on radioactivity and isotopes.

He was born in Warsaw and educated at Leipzig, Heidelberg, Zurich and Manchester. He worked in Germany from 1911 to 1935 and was director of the Munich Institute of Physical Chemistry.

In inorganic chemistry, he formulated what became known as Fajans rules to account for the formation of covalent rather than ionic bonds.

In 1913, independently but simultaneously with Frederick Soddy, he arrived at a theory of isotopes, and used this to explain the radioactive decay of Uranium-238. Also in 1913, along with Otto H. Gohring that he identified the element Protactinium.

He also used radioactivity to estimate the ages of minerals.

He died in Ann Arbor, Michigan, USA.

Bibliography

• 1913 - Radioactive Transformations and the Periodic System of the Elements
• ? - Application of the resonance theory to the structure of the water molecule
• 1941 - Artificial radioactive isotopes of Thallium, Lead and Bismuth
• 1948 - Electronic structure of molecules