Dymitrij Mendeleev, was the great Russian chemist, who lived in the year 1834-1907. Writing a handbook for students, setting elements in the order by their atomic mass he discovered a simply relation - every eight elements had similar features. So he put the elements into eight columns. He didn't fill up all gapes because some of the elements were still unknown. In the year 1875 gallium, the new element was discovered. Not long later scandium and germanium were discovered too. Their subsistence and features Mendeleev predicted before. That was the proof that he was right making his table.