Mendeleev use the atomic weights of the elements as the main property to classify the elements. He realised that some of the elements did not fit in with his scheme of classification as the order of atomic weight was strictly followed. In such cases, he ignored the order of atomic weights. For example, iodine with lower atomic weight (= 126.9) than that of tellurium (= 127.6) (Group (VI) was placed in Group VII along with other halogens because of similarities in properties.