Mendeleev used the atomic weights of the elements as the primary basis to classify them in his periodic table. He arranged the elements in order of their increasing atomic weights so that elements with similar properties occupied the same vertical column or group.

However, he did not strictly stick to this order of atomic weights. He realized that some elements did not fit in with his scheme if the order of atomic weight was strictly followed. Therefore, he ignored the order of atomic weights in certain cases, assuming the atomic measurements might be incorrect, and placed elements with similar properties together. For example, iodine, which has a lower atomic weight than tellurium, was placed in Group VII along with fluorine, chlorine, and bromine due to similarities in their properties.