Dmitri Ivanovich Mendeleev

Dmitri Ivanovich Mendeleev (1834–1907) was born in Tobolsk, Siberia. Educated at Saint Petersburg, he became professor of chemistry at Saint Petersburg State University. In 1869, while writing his textbook Principles of Chemistry, he sought to organize the known chemical elements according to their atomic weights and recurring valence properties.

He arranged the elements into a tabular grid, discovering the periodic law. Where elements seemed to be missing, Mendeleev boldly left gaps and predicted the exact atomic weights, densities, and chemical reactions of unknown elements—such as gallium (eka-aluminum), scandium (eka-boron), and germanium (eka-silicon). Within two decades, the discovery of these elements confirmed his predictions, cementing the Periodic Table as the foundation of modern chemistry.