Which of the following molecules or ions has a trigonal pyramid shape? (Molecular Geometry) Select one: a. KrF4 b. NH4+ c. O3 d. SeO32– e. COCl2
The Correct Answer and Explanation is:
ChatGPT said:
Correct Answer: d. SeO₃²⁻
Explanation:
To determine the molecular geometry (shape) of a molecule or ion, we apply the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory helps us predict the shape based on the repulsion between electron groups (bonding and lone pairs) around the central atom.
Let’s analyze each option:
a. KrF₄ (Krypton tetrafluoride)
- Central atom: Kr
- Kr has 8 valence electrons
- Forms 4 bonds with F and has 2 lone pairs
- Total regions of electron density = 6 → Octahedral electron geometry
- With 2 lone pairs, the molecular shape is square planar, not trigonal pyramidal.
b. NH₄⁺ (Ammonium ion)
- Central atom: N
- Forms 4 bonds with H
- No lone pairs on N
- Total electron regions = 4 → Tetrahedral electron and molecular geometry
- This is not trigonal pyramidal.
c. O₃ (Ozone)
- Central atom: One of the oxygen atoms
- Has resonance structures with a lone pair and double bond character
- Bent or angular shape
- Not trigonal pyramidal
d. SeO₃²⁻ (Selenite ion)
- Central atom: Se
- Has 6 valence electrons
- Forms 3 bonds with O atoms and has 1 lone pair
- Total regions = 4 → Tetrahedral electron geometry
- With 1 lone pair, the shape becomes trigonal pyramidal
This matches the definition of trigonal pyramidal: three atoms bonded to a central atom and one lone pair on the central atom.
e. COCl₂ (Phosgene)
- Central atom: C
- Forms 2 bonds with Cl and 1 double bond with O
- No lone pairs on C
- Total regions = 3 → Trigonal planar shape
- Not trigonal pyramidal
Final Answer: d. SeO₃²⁻ is the molecule with a trigonal pyramidal molecular geometry.
