Multiple Choice
Which molecular geometry is associated with a bond angle of approximately 109.5° around the central atom?
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12. Molecular Shapes & Valence Bond Theory
Molecular Geometry
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Which of the following molecules has a trigonal planar molecular geometry?
A
NH_3
B
H_2O
C
BF_3
D
CH_4
Verified step by step guidance1
Recall that molecular geometry depends on the number of bonding pairs and lone pairs around the central atom, as described by the VSEPR (Valence Shell Electron Pair Repulsion) theory.
Identify the central atom and count the number of bonding pairs and lone pairs of electrons for each molecule:
- NH_3 (ammonia): central atom is N, with 3 bonding pairs and 1 lone pair.
- H_2O (water): central atom is O, with 2 bonding pairs and 2 lone pairs.
- BF_3 (boron trifluoride): central atom is B, with 3 bonding pairs and 0 lone pairs.
- CH_4 (methane): central atom is C, with 4 bonding pairs and 0 lone pairs.
Use VSEPR theory to determine the molecular geometry for each molecule based on the electron pairs:
- Molecules with 3 bonding pairs and 0 lone pairs around the central atom adopt a trigonal planar geometry.
- Molecules with lone pairs will have geometries distorted from ideal shapes (e.g., trigonal pyramidal or bent).
Therefore, BF_3, with 3 bonding pairs and no lone pairs, has a trigonal planar molecular geometry.
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