Early periodic tables were incomplete, since many elements were unknown. Also, some elements were placed in groups with elements that were not similar to them. Mendeleev left gaps in his table for ...

A century and a half ago, a Russian chemistry professor published a classification of all the known elements, organized by atomic weight. Today, the system that he created for his students — plus some ...

he left gaps for yet to be discovered elements he switched the order of a few elements to keep the groups consistent Iodine has a lower relative atomic mass than tellurium. So iodine should be placed ...