Fajans, Kasimir (1887–1975)

Polish-born US chemist. He did pioneering work on radioactivity and isotopes, he also formulated rules that help to explain valence and chemical bonding.

In 1913, Fajans arrived simultaneously with, but independently of, Frederick Soddy at the theory of isotopes. He used this to explain how the decay of uranium-238, by emitting first an alpha particle and then two beta particles, produces a new uranium isotope four mass units lighter: uranium-234.

In inorganic chemistry, Fajans formulated two rules to account for the diagonal similarities between elements in the periodic table in terms of the ease of formation of covalencies and electrovalencies (ionic valencies). The first rule states that covalencies are more likely to be formed as the number of electrons to be removed or donated increases, so that highly charged ions are rare or impossible. The second rule states that electrovalencies are favoured by large cations and small anions.

Fajans also used radioactivity to estimate the ages of minerals.

Fajans was born in Warsaw and educated at Leipzig, Heidelberg, Zürich, and Manchester. He worked in Germany 1911–35, becoming director of the Munich Institute of Physical Chemistry. In 1936, he emigrated to the USA and served as a professor at the University of Michigan, Ann Arbor.