Question

In which of the following sets, the central atom of both the molecules does not obey the octet rule? \[ \text{I. } \mathrm{SF_4,\ XeF_4} \] \[ \text{II. } \mathrm{SCl_2,\ H_2S} \] \[ \text{III. } \mathrm{SnCl_2,\ XeO_3} \] The correct answer is (only):

• A. I, II, III
• B. I, III only
• C. II, III only
• D. I, II only

Hint:

Species such as \(\mathrm{SF_4}\), \(\mathrm{XeF_4}\), and \(\mathrm{XeO_3}\) exhibit expanded octets, whereas \(\mathrm{SnCl_2}\) is an example of an incomplete octet compound.

Answer 1

The Correct Option is B

Concept: The octet rule states that atoms tend to attain eight electrons in their valence shell. However, there are important exceptions:
• Expanded octet species (more than 8 electrons)
• Incomplete octet species (less than 8 electrons)
• Odd-electron species We must examine the central atom in each molecule.

Step 1: Analyze Set I. \[ \mathrm{SF_4} \] Sulphur has four bond pairs and one lone pair. Total electrons around sulphur: \[ 4\times2+2=10 \] Thus sulphur has an expanded octet. \[ \mathrm{XeF_4} \] Xenon has four bond pairs and two lone pairs. Total electrons around xenon: \[ 4\times2+4=12 \] Thus xenon also has an expanded octet. Therefore both molecules violate the octet rule. Set I is correct.

Step 2: Analyze Set II. \[ \mathrm{SCl_2} \] Sulphur forms two bonds and possesses two lone pairs. \[ 4+4=8 \] Octet is satisfied. \[ \mathrm{H_2S} \] Sulphur again has two bond pairs and two lone pairs. Total electrons around sulphur: \[ 8 \] Octet is satisfied. Therefore Set II is incorrect.

Step 3: Analyze Set III. \[ \mathrm{SnCl_2} \] Tin possesses one lone pair and two bond pairs. Total electrons around tin: \[ 6 \] Thus it has an incomplete octet. \[ \mathrm{XeO_3} \] Xenon forms three double bonds and possesses one lone pair. The valence shell contains more than eight electrons. Hence xenon has an expanded octet. Therefore both molecules violate the octet rule. Set III is correct.

Step 4: Select the correct answer. Only Sets I and III satisfy the condition. \[ \boxed{\text{I and III only}} \]