Trigonal Planar vs. Trigonal Pyramidal: Geometry & Bond Angles Explained
Trigonal planar molecules have one central atom bonded to three others with no lone pairs, forming a flat triangle at 120° angles. Trigonal pyramidal molecules also have three bonds, but one lone pair pushes the shape into a pyramid with ~107° angles.
Students mix them up because both involve three bonds around a central atom. In labs, confusing NH₃ (pyramidal) with BF₃ (planar) can lead to wrong solvent choices or misread spectra.
Key Differences
Planar: 120°, flat, no lone pairs, examples BF₃, AlCl₃. Pyramidal: ~107°, pyramid, one lone pair, examples NH₃, PCl₃. Electron repulsion from the lone pair compresses bond angles and creates the 3-D shape.
Which One Should You Choose?
Choose trigonal planar for symmetrical, non-polar molecules in catalyst design. Pick trigonal pyramidal when polarity and lone-pair reactivity matter, like in ammonia synthesis or biological amines.
Why do bond angles shrink from 120° to 107°?
The lone pair repels bonding pairs more strongly, compressing the angles.
Can a molecule switch shapes?
Yes, if the lone pair becomes a bond or vice versa, the geometry flips.