The first meaningful and remarkable contribution in the field of classification of elements was done by Russian Chemist by the name Dmitri I. Mendeleev in the year of 1869 and he proposed a law called Mendeleev periodic law which says that the physical & the chemical properties of the elements are a periodic function of their atomic masses.

On the foundation of this law, he developed a periodic table which was named as Mendeleev Periodic table, where he arranged all the elements in his periodic table in terms of their atomic weight or mass. In the table he made vertical columns called groups and vertical rows called as periods for the arranging the elements. He placed the elements with similar physical and chemical properties in the same group.

Nevertheless, he did not adhere to this system for long. He observed that if the elements were arranged strictly in terms of their increasing atomic weights, then some elements could not fit within this system of categorization.

Therefore, the order of atomic weights was ignored in some cases. For instance, the atomic weight of iodine is lesser than that of tellurium. Then to tellurium [in Group VI] was placed before iodine [in Group VII] just because iodine’s properties are so similar to that of fluorine, chlorine, and bromine. Also, argon was placed before potassium.