Question

Which pais have ability to form \( \text{p}\pi - \text{p}\pi \) multiple bonds

• A. C and O
• B. B and N
• C. N and P
• D. F and Cl
• E. C and Si

Hint:

Elements of the second period (C, N, O) uniquely form stable \( p\pi - p\pi \) multiple bonds. Elements in the third period and below generally avoid \( p\pi - p\pi \) bonding due to poor lateral overlap of their larger, more diffuse orbitals.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The ability of elements to form strong \( p\pi - p\pi \) multiple bonds (like double or triple bonds) depends heavily on their atomic size.
Small atomic size allows for effective lateral overlap of the \( p \)-orbitals.
Step 2: Key Formula or Approach:
The approach is to identify elements belonging to the 2nd period of the periodic table, as they possess small sizes and compact \( 2p \) orbitals, making \( p\pi - p\pi \) overlap highly effective.
Step 3: Detailed Explanation:
Let's analyze the given pairs:
(A) C and O: Both are 2nd-period elements with small atomic radii. They effectively overlap their \( 2p \) orbitals to form stable \( p\pi - p\pi \) double bonds, as seen in molecules like \( \text{CO}_2 \), ketones, and aldehydes.
(B) B and N: Although both are 2nd period, typical multiple bonding is less dominant compared to Carbon and Oxygen, though they do form \( p\pi - p\pi \) in specific aromatic-like rings (e.g., borazine). However, C and O is the classic and most prolific pair for this.
(C) N and P: Phosphorus is a 3rd-period element. Its \( 3p \) orbitals are larger and more diffuse, resulting in weak \( p\pi - p\pi \) overlap.
(D) F and Cl: Halogens typically form single bonds. Chlorine is a 3rd-period element and does not favor \( p\pi - p\pi \) bonding.
(E) C and Si: Silicon is a 3rd-period element. Its larger size makes \( p\pi - p\pi \) overlap with carbon very weak (which is why \( \text{CO}_2 \) is a gas with double bonds, while \( \text{SiO}_2 \) forms a solid network of single bonds).
Thus, Carbon and Oxygen are the best and most universally recognized pair for forming robust \( p\pi - p\pi \) multiple bonds.
Step 4: Final Answer:
C and O have the ability to form \( p\pi - p\pi \) multiple bonds.