A Russian chemist, Dmitri Mendeleev was the first to develop a periodic table & he gave a law called Mendeleev periodic law which states that the physical & chemical properties of the elements are a periodic function of their atomic masses.On the basis of this law he developed a Mendeleev periodic table, where he  arranged the elements in his periodic table ordered by atomic weight or mass. He arranged the elements in periods and groups in order of their increasing atomic weight. He placed the elements with similar properties in the same group.

However, he did not stick to this arrangement for long. He found out that if the elements were arranged strictly in order of their increasing atomic weights, then some elements did not fit within this scheme of classification.

Therefore, he ignored the order of atomic weights in some cases. For example, the atomic weight of iodine is lower than that of tellurium. Still Mendeleev placed tellurium (in Group VI) before iodine (in Group VII) simply because iodine’s properties are so similar tofluorine, chlorine, and bromine.