【正确答案】The designation of iron, cobalt, nickel, and the six "platinum elements" as one periodic group was a historical accident. Tile properties of the three elements in each period are rather similar, and Mendeleev considered their different from any other group(the noble gases had not yet been discovered. ) If it were to be done again, there is little reason to suppose that the nine elements would not be put into three groups-"VII, IX, and X "