Question

A molecule ‘X’ has square pyramidal geometry, as per VSEPR theory. The number of bond pairs and lone pairs of electrons present in the valence shell of central atom of X, respectively, are

• A. 4, 1
• B. 5, 1
• C. 5, 2
• D. 6, 1

Hint:

In VSEPR theory, square pyramidal geometry arises from 6 electron domains (octahedral arrangement) with 5 bond pairs and 1 lone pair. Count the total electron domains first, then determine the geometry based on the number of lone pairs.

Answer 1

The Correct Option is B

Step 1: Understand Square Pyramidal Geometry in VSEPR Theory
In VSEPR (Valence Shell Electron Pair Repulsion) theory, the geometry of a molecule is determined by the number of electron domains (bond pairs and lone pairs) around the central atom and their arrangement to minimize repulsion. A square pyramidal geometry arises when the central atom has a specific arrangement of electron domains. Step 2: Determine the Number of Electron Domains for Square Pyramidal Geometry
Square pyramidal geometry is typically observed when the central atom has 6 electron domains (steric number = 6), which are arranged in an octahedral manner. In an octahedral arrangement:
There are 6 positions around the central atom, each occupied by an electron domain (either a bond pair or a lone pair).
For a square pyramidal shape, 5 of these positions are occupied by bond pairs (attached to surrounding atoms), and 1 position is occupied by a lone pair.
This lone pair distorts the octahedral geometry, removing one vertex and leaving a square base with one atom at the apex, forming a square pyramid. Step 3: Identify Bond Pairs and Lone Pairs
In an octahedral arrangement with 6 electron domains, if one domain is a lone pair, the remaining 5 domains must be bond pairs to form the square pyramidal shape.
Thus, the central atom has:
5 bond pairs (corresponding to 5 surrounding atoms).
1 lone pair (causing the distortion from octahedral to square pyramidal).
So, the number of bond pairs and lone pairs is 5 and 1, respectively. Step 4: Verify with a Common Example
A common molecule with square pyramidal geometry is \( \text{IF}_5 \):
Iodine (I) is the central atom with 7 valence electrons.
It forms 5 bonds with fluorine atoms, using 5 of its valence electrons, leaving 2 electrons (1 lone pair).
Total electron domains = 5 bond pairs + 1 lone pair = 6, which aligns with an octahedral arrangement.
The lone pair occupies one position, and the 5 bond pairs form the square pyramidal shape.
This confirms our understanding: 5 bond pairs and 1 lone pair. Step 5: Analyze Options
Option (1): 4, 1. Incorrect, as 4 bond pairs and 1 lone pair (5 electron domains) would lead to a trigonal bipyramidal arrangement, resulting in a seesaw shape, not square pyramidal.
Option (2): 5, 1. Correct, as 5 bond pairs and 1 lone pair (6 electron domains) lead to a square pyramidal geometry, as explained.
Option (3): 5, 2. Incorrect, as 5 bond pairs and 2 lone pairs (7 electron domains) would lead to a pentagonal bipyramidal arrangement, resulting in a pentagonal pyramidal shape, not square pyramidal.
Option (4): 6, 1. Incorrect, as 6 bond pairs and 1 lone pair (7 electron domains) also lead to a pentagonal bipyramidal arrangement, not square pyramidal.