Question

What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev’s periodic law.

Answer 1

Mendeleev was the most important contributor to the early development of the periodic table. In his periodic table, elements were arranged on the basis of the fundamental property, atomic mass and chemical properties.

Complete answer:
Mendeleev found that the properties of elements are related to atomic mass in a periodic manner. He arranged the elements such that the elements having similar properties fell into the same vertical columns of the periodic table. The physical property that he used was the atomic mass of an element. The chemical property he used was the nature of oxide and hydride formed i.e. similarity in chemical properties were used by him. The two observations that posed a challenge in Mendeleev periodic law were: Arranging elements according to the increasing order of their atomic masses could not be maintained. Chemical properties do not depend on the atomic mass of an element. And isotopes were not given any place in the table as they have different atomic mass but same chemical properties.

Note:
Some gaps were left in Mendeleev’s periodic table for the elements yet to be discovered. So, if a new element is discovered, it can be placed in a new group without disturbing the existing groups. But he was unable to locate hydrogen in the periodic table. The isotopes discovered later violated Mendeleev’s periodic law.