Question

What are the limitations of Mendeleeff 's periodic table?

Short answer

Answer: The limitations of Mendeleev's periodic table include the improper placement of hydrogen, anomalies in atomic mass, inability to accommodate isotopes, exclusion of lanthanides and actinides, and failure to predict noble gases.

Step-by-step solution

1. Position of Hydrogen

Mendeleev's periodic table could not assign a proper position for hydrogen. Hydrogen has characteristics of both alkali metals and halogens, but it could not be placed in both groups simultaneously, so its placement remains an issue.

2. Anomaly in Atomic Mass

Mendeleev placed elements in the periodic table based on their increasing atomic mass. However, there were cases when elements with a lower atomic mass were placed before elements with a higher atomic mass. For example, cobalt (atomic mass 58.93) comes before nickel (atomic mass 58.69), which contradicts Mendeleev’s principle.

3. Position of Isotopes

Mendeleev's periodic table did not take into account isotopes, i.e., variations of elements with different atomic masses but the same atomic number. Isotopes cannot be accommodated in Mendeleev's periodic table as they would require additional spaces, leading to anomalies in the arrangement of elements.

4. Lanthanides and Actinides

Mendeleev's periodic table did not have space for the inclusion of the lanthanide and actinide series. These elements were discovered later, and their properties did not fit with Mendeleev's original arrangement.

5. Prediction of Elements

Although Mendeleev managed to predict some undiscovered elements by leaving gaps in his periodic table, his table failed to predict the existence of the noble gases. The discovery of noble gases led to the formulation of a new group in the periodic table. These were the limitations present in Mendeleev's periodic table, which were eventually addressed with the development of the modern periodic table.