Dmitri Ivanovich Mendeleev in his study at home in 1904. Image provided by Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library. Dmitri Mendeleev (1834–1907) Born in the Siberian town of Tobolsk, Dmitri Mendeleev was the youngest of 17 children. His father died when he was still in his early teens, and his mother operated a glass factory to support the family. Dmitri managed to continue his studies, focusing on chemistry. In the 1860s, while teaching at the University of St. Petersburg, Mendeleev set out to write his own chemistry textbook. It was while researching and writing this text that he began to investigate the relationships between the properties of different elements. The result was the periodic table, in which elements with similar properties were found to fall into columns when arranged according to increasing atomic mass (the concept of atomic numbers had not yet been developed). Blank spaces occurred in some of Mendeleev’s columns, and he boldly predicted that new elements would be discovered to fill them, a prediction that came true in time. In addition to his periodic table, Mendeleev also did practical work to help develop Russian agriculture and the Russian oil industry. A political progressive, he often had trouble with the Imperial government. And in case you're wondering, Mendeleev had his hair cut once each year, whether he needed it or not.