Question

Which was a defect of Mendeleev's periodic table ?

• A. Not giving proper place to oxygen
• B. Not giving proper place to Cl
• C. Not giving proper place to hydrogen
• D. None of these

Hint:

The three major limitations of Mendeleev's periodic table can be remembered as issues with Hydrogen, Isotopes, and Pairs (anomalous pairs).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Mendeleev's periodic table, while revolutionary for its time, had certain limitations or defects. These were later addressed by the modern periodic table.

Step 2: Detailed Explanation:
The main defects of Mendeleev's periodic table were:
\begin{enumerate} \item Position of Hydrogen: Hydrogen shows properties similar to both alkali metals (Group 1) and halogens (Group 17). It forms a +1 ion like alkali metals and exists as a diatomic molecule (\(H_2\)) like halogens (\(F_2, Cl_2\)). Due to this dual nature, Mendeleev could not assign a fixed, correct position for hydrogen in his table.
\item Position of Isotopes: Isotopes are atoms of the same element with the same atomic number but different mass numbers. Since Mendeleev's table was based on increasing atomic mass, isotopes would have required different positions, which would disrupt the periodic arrangement.
\item Anomalous Pairs: In some cases, Mendeleev placed elements with a slightly higher atomic mass before elements with a lower atomic mass to maintain the order of properties (e.g., Cobalt before Nickel).
\end{enumerate} Among the given options, not giving a proper place to hydrogen is a well-known defect.

Step 3: Final Answer:
A major defect of Mendeleev's periodic table was its failure to assign a proper and fixed place to hydrogen.