CBSE Class 10 Answered

The creation of Mendeleev's periodic table was based upon certain physical and chemical properties:

Physical properties: The atomic masses of the elements were taken into account and the elements were arranged in order of increasing atomic masses. This influenced some of their physical properties like melting points, boiling points, density etc.

Chemical properties: The distribution of the elements into different groups was linked with formation of hydrides by combining with hydrogen and formation of oxides by combining with oxygen. This is linked with the valency of the elements.

The two main observations which posed challenge to Mendeleev's periodic table are as follows:

(i) Position of isotopes: Since the isotopes of an element differ in their atomic masses, they must be assigned separate positions in the periodic table.

(ii) Anomalous positions of some elements: In the Mendeleev's periodic table, certain elements with higher atomic masses precede or placed before the elements with lower atomic masses. For example: the elements Argon (At. mass = 39.9) is placed before the element Potassium (At. Mass = 39.1).