AIIMS 2005 Chemistry Silicon Compounds Assertion Reason Question

To tackle the given question effectively, we will analyze both the assertion and the reason, and then evaluate the options provided.

Assertion and Reason Analysis

Assertion: SiF₆²⁻ is known but SiCl₆²⁻ is not.

Reason: The size of fluorine is small, and its lone pair of electrons interacts with the d-orbitals of Si strongly.

Explanation of the Correct Answer

1. Understanding the Assertion:

   • SiF₆²⁻ (silicon hexafluoride anion) is indeed a known and stable species. It forms when silicon, which has a +4 oxidation state, interacts with six fluoride ions. The strong electronegativity and small size of fluorine allow for effective overlap of orbitals, facilitating the formation of a stable octahedral geometry around silicon.
   • In contrast, SiCl₆²⁻ (silicon hexachloride anion) is not known. Chlorine is larger than fluorine, leading to less effective orbital overlap and weaker interactions with silicon's d-orbitals. The larger size of chlorine makes it less favorable for forming stable silicon complexes compared to fluorine.

2. Understanding the Reason:

   • The reason given highlights the significance of fluorine's small size. The small atomic radius of fluorine allows for strong interactions with silicon's d-orbitals due to better overlap. This strong interaction stabilizes SiF₆²⁻ due to the effective formation of bonds.
   • Thus, the reason correctly explains why SiF₆²⁻ is stable and why SiCl₆²⁻ is not. The lone pairs of electrons on fluorine can engage in back-bonding with silicon's vacant d-orbitals, further stabilizing the structure.

Conclusion on the Correct Option

Since both the assertion and reason are true, and the reason appropriately explains the assertion, the correct answer is:

A) If both assertion and reason are true and reason is the correct explanation of assertion

Clarification of Other Options

• B) If both assertion and reason are true but reason is not the correct explanation of assertion: This option is incorrect because the reason actually provides a valid explanation for the stability of SiF₆²⁻ compared to SiCl₆²⁻.

• C) If assertion is true but reason is false: This option is also incorrect since both the assertion and reason are true.

• D) If both assertion and reason are false: Clearly incorrect, as we have established that both are true.

Summary

In summary, the stability of SiF₆²⁻ can be attributed to the small size of fluorine, which allows for strong interactions with silicon's d-orbitals, leading to effective bonding and a stable anion. This contrasts with SiCl₆²⁻, which fails to form due to less effective orbital interactions. Thus, option A is justified as both statements are valid and the reason correctly explains the assertion.