Born in Warsaw, Poland, in 1887, Fajans was educated in Germany and received a Ph. D from the University of Heidelberg in 1909. He did postdoctoral studies with Richard Willstater at Zurich University and with Ernest Rutherford in Manchester, England. In 1911 he joined the staff of Karlsruhe Institute of Technology. From 1917 to 1935 he was a professor at the University of Munich, serving as the Director of the Institute of Physical Chemistry from 1932 to 1935. He was also director of the Institute for Physico-Chemical Measurements.
Before coming to the University of Michigan in 1936, Fajans had established an international reputation for his scientific achievements in many fields of both chemistry and physics. He was already an authority on many phases of radioactivity, refractivity, adsorption, molecular structure, crystal structure, photochemistry, and thermochemistry. In the early days of his studies in radioactivity, while working at Manchester, Karlsruhe, and Munich, Fajans was one of the first to develop the idea of matter as composed of isotopes of the several elements, and to make clear the nature of the transformation in the natural radioactive series. Fajans' studies on radioactivity established the radioactive displacement laws (Fajans-Soddy Displacement Laws) and helped place radio elements in the periodic system. The Fajans-Paneth-Hahn rule resulted from his work on the precipitation and absorption of radio elements and dye-ions. With Oswald Göhring, he discovered the Uranium x2 form of protactinium-234. His quanticule theory of chemical binding was the focus of his later work. Fajans retired from the university in 1957. He died in 1975.