Mendeleev made atomic weight as the basis of classification of elements in periodic table. He arranged the elements in increasing order of atomic masses and noticed that elements with similar properties were present in one group and similarly was also found in properties of compounds formed by these elements. But there were some exceptions in increasing order of atomic weight. Therefore, the increasing order of atomic weight was not completely followed, then importance was given to similarity in properties. Like even though the atomic weight of I was less than Te (group VI), it was placed in group VII along with F, Cl. Thus, he could not stick to the increasing order of atomic weight in classification of elements.