Question

What are the demerits of Mendeleev’s periodic table with respect to the arrangement of elements in ascending order? (Any three)

Answer 1

Hint: Try to solve the question by defining Mendeleev’s periodic table and state out the basis of the periodic table. Then summarize the demerits with respect to the arrangement of elements in ascending order.

Complete step by step answer:
Mendeleev published his periodic table in 1869. His arrangement of elements was based on atomic mass. Mendeleev’s periodic table contributed to predict properties of elements that had not been discovered then. Along with some merits, there were some demerits in it.
The demerits of Mendeleev’s periodic table with respect to the arrangement of elements in ascending order are given below –

1) Hydrogen's position
This is the most important point. Hydrogen resembled the properties of both alkali metals and halogens. Therefore, the position of hydrogen (if hydrogen should be placed with halogens or alkali metals) was not specified.

2) Position for Isotopes
Isotopes are the elements with the same atomic number and different mass number. Mendeleev’s periodic table was based on representing elements in increasing order of atomic masses. But isotopes were not included in his periodic table.

3) Some elements were arranged in the reverse order. Elements with a higher atomic mass were placed in before the elements with less atomic mass. For example - Cobalt and Nickel. Cobalt has an atomic mass of 58.93 whereas nickel has an atomic mass of 58.71. Since elements were placed because of their atomic masses, Cobalt being heavier was placed before Nickel, in the Mendeleev's periodic table.

Note: Mendeleev formulated a periodic law which states:“the properties of elements are the periodic function of their atomic masses.”