AIIMS 2005 Chemistry VSEPR Theory MCQ Question

To determine which of the given molecules have the same number of lone pairs on the central xenon (Xe) atom, we will analyze each molecule's structure using the Valence Shell Electron Pair Repulsion (VSEPR) theory.

Step 1: Determine the Valence Electrons for Xenon

Xenon (Xe) is a noble gas with the atomic number 54, and it has the following electron configuration:

$$[Xe] = 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 4p^6 4d^{10} 5s^2 5p^6$$

In its outer shell, Xe has 8 valence electrons.

Step 2: Analyze Each Molecule:

1. (i) XeO₃:

   • In XeO₃, xenon is bonded to three oxygen atoms.
   • The molecular structure suggests that each oxygen forms a double bond with xenon (considering the formal charges and stability).
   • Thus, the bonding involves 6 electrons (3 double bonds), leaving 2 electrons as lone pairs on Xe.
   • Lone Pairs on Xe in XeO₃: 2

2. (ii) XeOF₄:

   • In XeOF₄, xenon is bonded to one oxygen atom and four fluorine atoms.
   • The oxygen atom usually forms a double bond with xenon, utilizing 4 electrons (2 from the double bond).
   • The four fluorine atoms each form single bonds with xenon, utilizing 4 additional electrons.
   • Therefore, 8 valence electrons are used for bonding, leaving no lone pairs on Xe.
   • Lone Pairs on Xe in XeOF₄: 0

3. (iii) XeF₆:

   • In XeF₆, xenon is bonded to six fluorine atoms.
   • Each fluorine forms a single bond with xenon, utilizing 6 electrons.
   • Thus, since all 8 valence electrons are used for bonding, Xe has 2 lone pairs.
   • Lone Pairs on Xe in XeF₆: 2

Summary of Lone Pairs:

• XeO₃: 2 lone pairs
• XeOF₄: 0 lone pairs
• XeF₆: 2 lone pairs

Conclusion:

Now we can summarize the lone pairs:

• XeO₃ and XeF₆ both have 2 lone pairs on xenon.
• XeOF₄ has 0 lone pairs on xenon.

Thus, the molecules that have the same number of lone pairs on Xe are XeO₃ and XeF₆.

Correct Answer:

• The correct choice is D) (i), (ii), and (iii). However, this seems to contradict our analysis. The actual conclusion should focus on XeO₃ and XeF₆ having the same number of lone pairs (2), while XeOF₄ has none (0).

Incorrect Options Analysis:

• Option A (i) and (ii) only: Incorrect because they do not have the same number of lone pairs.
• Option B (i) and (iii) only: Incorrect because although XeO₃ and XeF₆ have the same lone pairs, XeOF₄ does not.
• Option C (ii) and (iii) only: Incorrect because XeOF₄ has 0 lone pairs, while XeF₆ has 2.

Final Clarification:

The correct options would ideally be (i) and (iii) as they share the same number of lone pairs (2), thus it appears that the answer provided in the question may need revision. The correct conclusion based on the shared lone pairs should be highlighted as (i) and (iii) only.

In summary, the analysis concludes that XeO₃ and XeF₆ both have 2 lone pairs, while XeOF₄ has none.