Question

Identify the pair of molecules in which one of them is a molecule with an odd electron and the other has an expanded octet.

• A. BeCl₂ & HNO₃
• B. NO & PF₅
• C. BCl₃ & NO₂
• D. SCl₂ & NH₃

Hint:

When identifying molecules with expanded octets, remember that elements in Period 3 and beyond (such as Phosphorus, Sulfur, etc.) can use d-orbitals to accommodate more than 8 electrons. Molecules with odd numbers of electrons are typically free radicals, such as NO and NO₂.

Answer 1

The Correct Option is B

The problem asks us to identify a pair of molecules, where one molecule has an odd number of electrons (a free radical), and the other has an expanded octet. 

Let's examine the given options: 
- Option A: BeCl₂ & HNO₃ - BeCl₂ has no unpaired electrons (Be is in Group 2 and typically forms stable molecules with paired electrons). - HNO₃ is a stable molecule with no free radicals and does not have an expanded octet. Hence, this pair doesn't satisfy the given conditions. 
- Option B: NO & PF₅ - NO (Nitric oxide) is a free radical with an odd number of electrons, specifically one unpaired electron. - PF₅ (Phosphorus pentafluoride) has an expanded octet. Phosphorus can accommodate more than 8 electrons because it is in Period 3 and can use d-orbitals for bonding. This option satisfies the condition of one molecule with an odd electron and the other with an expanded octet. 
- Option C: BCl₃ & NO₂ - BCl₃ has no free radicals and follows the octet rule (boron typically forms 3 bonds with no lone pairs). - NO₂ is a molecule with an odd electron (it has one unpaired electron and is a free radical). However, it does not have an expanded octet. Thus, this option doesn't fit the required condition. 
- Option D: SCl₂ & NH₃ - SCl₂ follows the octet rule and has no free radicals. - NH₃ (Ammonia) follows the octet rule and also has no free radicals. Hence, this option does not meet the criteria. Therefore, the correct pair is NO & PF₅, which fits the condition of one molecule having an odd electron and the other having an expanded octet.