CBSE Class 10 Answered

Merits of Mendeleev's periodic table:

1. It simplified the study of elements. As the arrangements of elements showing similar properties were classified into groups, it was very useful in studying and remembering the properties of a large number of elements in a systematic way.

2. Based on the positions in the periodic table, Mendeleev could predict the properties of some undiscovered elements.There were many vacant spaces in his table for the elements to be discovered. He named them Eka-Boron, Eka-Aluminium and Eka-silicone. He also predicted the properties of these undiscovered elements including atomic masses.

3. Mendeleev’s arrangement helped in correcting the atomic masses of some of the elements, based on their positions in the periodic table.

Demerits of Mendeleev's periodic table:

1. The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).
2. In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses, while some dissimilar elements were grouped together. Thus, the position of these elements was not justified.
3. In certain pairs of elements like, Ar (40) and K (39) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.
4. According to Mendeleev's periodic table these should be placed at three separate places. However isotopes have not been given separate places in the periodic table.