Question

What are the limitations of Mendeleev’s periodic table? How could the modern periodic table overcome the limitations of the Mendeleev table?

Answer 1

To answer this question, try to solve by defining Mendeleev’s and modern periodic table and state out the basis of the periodic table. Then summarize the limitations of the Mendeleev’s periodic table with respect to the arrangement of elements and then explain about the differences, the modern table has with respect to Mendeleev’s periodic table.

Complete step by step solution:
Mendeleev published his periodic table in the year $1869$. His arrangement of elements was based on the 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 limitations in it.
The limitations of Mendeleev’s periodic table are given below:
- Hydrogen’s position
This is the most important point. Hydrogen resembled the properties of both alkali metals and halogens. Therefore, the position of hydrogen (whether it should be placed with halogens or alkali metals) was not specified.
- Position for Isotopes
We know what isotopes are. Isotopes are the elements with the same atomic number and different mass number. Mendeleev’s periodic table was based on representing elements in the increasing order of atomic masses. But isotopes were not placed in his periodic table.
- Some elements were arranged in the reverse order in his periodic table. 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 $58.71$. Since, elements were placed because of their atomic masses, Cobalt being heavier was placed before Nickel in his table.
Thus, there was a need for a modern periodic table, which further helped to overcome these limitations. So, let’s see how the modern periodic table overcome these limitations:
- In Mendeleev’s periodic table, the noble elements were not included as they were not discovered at that time. In modern periodic tables, the noble elements were placed in groups as a last and separated group.
- In Mendeleev’s periodic table, elements with larger atomic masses were placed before the elements with smaller atomic weight without any justified reason. But in modern periodic tables, all were arranged systematically with increasing atomic number and weight too.
- In Mendeleev’s periodic table, some of the different elements were grouped together while some similar elements were placed in different groups. For example, alkali metals were placed with coinage metals. But in the modern periodic table, there was no such confusion. The coinage metals were placed in the d-block and alkali metals in s-block.
- In modern periodic tables, the isotopes were assigned the same position as they have the same atomic number.

Note: Mendeleev formulated a periodic law which states that- “the properties of the elements are the periodic function of their atomic masses”. Besides having demerits, it has contributed to predict properties of elements that had not been discovered then.