Between the years 1817 and 1869, several scientists, from Dobereiner to Mendeleev, devised different schemes for classifying the elements.
William Odling (1829-1921) in 1864 was among those who were intrigued by atomic weights and the periodic occurrence of chemical properties. He proposed repeating units of 7 (William Odling, 1864). The table of elements he drew up bears a striking resemblance to Mendeleev’s first table. The groups are horizontal, the elements are in order of increasing atomic weight and there are vacant slots for undiscovered ones. In addition, Odling overcame the tellurium iodine problem, and he even managed to get thallium, lead, mercury and platinum in the right groups - something that Mendeleev failed to do at his first attempt.