[1] After he had completed secondary school in Warsaw (1904), he studied chemistry in Germany, first at the University in Leipzig, and then in Heidelberg and Zürich.

Fajans worked with Henry G. Moseley at the laboratory of Ernest Rutherford researching properties of the radioactive rows of the periodic table.

[3] Fajans and Otto Hahn discovered the formula that defined the conditions of precipitation and absorption of radioactive substances.

On the basis of his research data Fajans formulated the essential relationships concerning chemical bond strength and deformation of ions and particles, such as heat of ion hydration, refractive index and the heat of sublimation.

In 1923 he formulated Fajans' rules of inorganic chemistry, which are used to predict whether a chemical bond will be covalent or ionic.