Mendeleev was Professor of Chemistry at the University of St. Petersburg (now Leningrad) in Russia. Mendeleev's work as a whole amounted to more than 400 books and articles as well as a large number of manuscripts which are preserved at the University of Leningrad. Mendeleev dedicated his life to research and education. Mendeleev made it his special responsibility to educate people wherever he went. When he traveled, Mendeleev would ride in the third class section of the train just to share findings with peasants over a cup of tea.
In 1869 Mendeleev arranged all the known elements (63 at that time) in order of increasing mass of the atoms. He found that the properties of elements repeated themselves at regular intervals. He arranged them in the form of a table so that athe elements with similar properties would fall into a vertical comumn. Because the properties of elements repeated themselves at regular intervals or periods the table is called the Periodic table of elements.
Mendeleev left gaps in the table to make the elements fit into proper columns and announced that the gaps represented elements not yet discovered. Mendeleev predicted the properties of the undiscovered elements by judging the properties of the elements above and below the gap in the table.