CBSE Class 10 Answered

Physical property - atomic mass

Chemical property - Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae
of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.

Observations which posed a challenge to mendleev's periodic law:

1. He could not assign a correct position to hydrogen in his Table.

2. Isotopes discovered long after Mendeléev had proposed his periodic classification of elements posed a challenge to his table.

3. Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements — especially when we consider the heavier elements.