What important property did Mendeleev used …

The important property that Mendeleev used for the classification of elements was atomic weight. He arranged all known elements in the form of periodic table. Some of the elements did not fit very well if the order of atomic weight was strictly followed. For these elements, he ignored the order of atomic weight (as atomic weight measurements can be incorrect) and placed the elements with similar properties together. Thus, iodine having lower atomic weight than tellurium (of group VI) was placed in group VII along with fluorine, chlorine and bromine due to similar properties and left some gaps for undiscovered elements.

 By considering the properties the adjacent elements, he predicted the properties of undiscovered elements. Later on, these elements were discovered and their properties were similar to those predicted by Mendeleev. 

Thus, Mendeleev left gaps for gallium and germanium. He named these elements as eka-aluminium and eka-silicon. When these elements were discovered, their properties were similar to those predicted by Mendeleev.