Question

Select the mismatch:

• A. $ \text{NH}_3 $ – Trigonal Pyramidal
• B. $ \text{H}_2\text{S} $ – Bent
• C. $ \text{CHCl}_3 $ – Trigonal Pyramidal
• D. All are correctly matched

Hint:

Remember to count both bonding pairs and lone pairs around the central atom to determine the electron geometry (steric number), and then use the arrangement of only the atoms (excluding lone pairs) to find the molecular geometry. Lone pairs exert more repulsion than bonding pairs.

Answer 1

The Correct Option is C

Step 1: Understand VSEPR Theory for determining molecular geometry.
Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the geometry of molecules based on minimizing the repulsion between electron pairs (both bonding and lone pairs) in the valence shell of the central atom. The steps involve determining the central atom, counting its valence electrons, identifying bonded atoms, calculating lone pairs, and then applying the VSEPR rules. Step 2: Determine the geometry for each given molecule.
Molecule: NH\(_3\) (Ammonia)
Central atom: Nitrogen (N)
Valence electrons on N: 5
Number of atoms bonded to N: 3 (Hydrogen atoms)
Number of lone pairs on N: (5 valence electrons - 3 bonding electrons) / 2 = 1 lone pair
Steric number (number of bonded atoms + lone pairs) = 3 + 1 = 4
Electron geometry (arrangement of electron groups): Tetrahedral
Molecular geometry (arrangement of atoms): The one lone pair exerts more repulsion than bonding pairs, pushing the three N-H bonds closer, resulting in a Trigonal Pyramidal molecular geometry.
Match Status: Correct.
Molecule: H\(_2\)S (Hydrogen Sulfide)
Central atom: Sulfur (S)
Valence electrons on S: 6
Number of atoms bonded to S: 2 (Hydrogen atoms)
Number of lone pairs on S: (6 valence electrons - 2 bonding electrons) / 2 = 2 lone pairs
Steric number = 2 + 2 = 4
Electron geometry: Tetrahedral
Molecular geometry: The two lone pairs exert significant repulsion on the two S-H bonding pairs, resulting in a Bent (or V-shaped) molecular geometry.
Match Status: Correct.
Molecule: CHCl\(_3\) (Chloroform)
Central atom: Carbon (C)
Valence electrons on C: 4
Number of atoms bonded to C: 4 (1 Hydrogen, 3 Chlorine atoms)
Number of lone pairs on C: (4 valence electrons - 4 bonding electrons) / 2 = 0 lone pairs
Steric number = 4 + 0 = 4
Electron geometry: Tetrahedral
Molecular geometry: With no lone pairs on the central carbon atom and four bonded atoms, the molecular geometry is Tetrahedral.
The given geometry is "Trigonal Pyramidal".
Match Status: Incorrect. Step 3: Identify the mismatch.
Based on the VSEPR theory analysis, the geometry provided for CHCl\(_3\) (Trigonal Pyramidal) is incorrect. The correct geometry for CHCl\(_3\) is Tetrahedral.