JEE MAIN - Chemistry (2024 - 5th April Evening Shift - No. 20)
Given below are two statements :
Statement I : The metallic radius of $$\mathrm{Na}$$ is $$1.86 \mathrm{~A}^{\circ}$$ and the ionic radius of $$\mathrm{Na}^{+}$$ is lesser than $$1.86 \mathrm{~A}^{\circ}$$
Statement II : Ions are always smaller in size than the corresponding elements.
In the light of the above statements, choose the correct answer from the options given below :
Explanation
To determine the correctness of the given statements, let's analyze each one individually based on atomic and ionic radii concepts.
Statement I:
The metallic radius of $\mathrm{Na}$ is $1.86\, \text{Å}$ and the ionic radius of $\mathrm{Na}^+$ is lesser than $1.86\, \text{Å}$.
Analysis:
Metallic Radius of Sodium ($\mathrm{Na}$):
Sodium is a metal, and its metallic radius is indeed approximately $1.86\, \text{Å}$.
The metallic radius refers to half the distance between the nuclei of two adjacent atoms in a metallic lattice.
Ionic Radius of Sodium Ion ($\mathrm{Na}^+$):
When sodium loses an electron to form $\mathrm{Na}^+$, it loses its outermost electron shell (the 3s orbital).
This results in a significant decrease in size due to:
Decrease in Electron-Electron Repulsion: Fewer electrons mean less repulsion among them.
Unchanged Nuclear Charge: The number of protons remains the same, so the effective nuclear charge per electron increases, pulling the remaining electrons closer to the nucleus.
The ionic radius of $\mathrm{Na}^+$ is approximately $0.95\, \text{Å}$, which is significantly smaller than $1.86\, \text{Å}$.
Conclusion:
Statement I is correct.
Statement II:
Ions are always smaller in size than the corresponding elements.
Analysis:
General Trends:
Cations ($\text{Positive Ions}$):
Formed by the loss of one or more electrons.
Result: Cations are smaller than their parent atoms due to loss of electron(s) and decreased electron-electron repulsion.
Example: $\mathrm{Na} \rightarrow \mathrm{Na}^+$ (size decreases).
Anions ($\text{Negative Ions}$):
Formed by the gain of one or more electrons.
Result: Anions are larger than their parent atoms due to added electron(s) and increased electron-electron repulsion.
Example: $\mathrm{Cl} \rightarrow \mathrm{Cl}^-$ (size increases).
Exceptions to Statement II:
Since anions are larger than their corresponding neutral atoms, the statement that "ions are always smaller in size than the corresponding elements" is incorrect.
Conclusion:
Statement II is false.
Final Answer:
Statement I is correct but Statement II is false.
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