atomic weight = equivalent weight x valency
The combining weights were generally accurate but sometimes an element was given the wrong valency. Thus beryllium, combining weight 4.6, was given the valency 3 because it was chemically similar to aluminum. This gave an atomic weight of 13.8, placing it between carbon and nitrogen where there was no space. Mendeleev said the valency was 2; the problem was solved - it fitted into the space between lithium and boron.
Mendeleev sometimes decided that atomic weights must be wrong because the elements simply appeared in the wrong place. For example he placed tellurium before iodine although its atomic weight is greater simply because iodine’s properties are so similar to those of fluorine, chlorine and bromine and tellurium’s to those of oxygen, sulfur and selenium rather than the other way round. We now know that it is atomic number, not relative atomic mass that governs an element’s position in the Periodic Table but in most cases the two result in the same order.The greatness of Mendeleev was that not only did he leave spaces for elements that were not yet discovered but he predicted properties of five of these elements and their compounds.