Beginner Topic 154 of 1102 Bonding & Molecular Structure
Trigonal Planar Geometry
Three groups at 120 degrees in a plane, as in BF3.
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In-depth explanation of Trigonal Planar Geometry
Trigonal planar geometry occurs when a central atom has three electron domains, which lie at 120 degrees to one another in a single plane. The central atom is sp2 hybridized, leaving one unhybridized p orbital available for pi bonding. Molecules with three bonding pairs and no lone pairs, such as boron trifluoride, adopt this shape. The same geometry describes the carbonyl carbon in aldehydes and ketones and the nitrate and carbonate ions. Because the bond dipoles are arranged symmetrically, a molecule with three identical bonds is nonpolar.
Going deeper: Trigonal Planar Geometry
Key ideas: For AX3 with no lone pairs: BF3, SO3, NO3-, CO3 2-, H2C=O (about 120 degrees; in formaldehyde the H-C-H angle is about 116 because the double bond takes more room). The central atom is sp2 with one p orbital for pi bonding. Sum of the three angles equals 360 degrees as all atoms lie in one plane.
Common mistakes and connections: Mistakes include assuming exactly 120 degrees when different groups are present, and confusing trigonal planar with trigonal pyramidal. Do not assume BF3 is polar. Also SO3 is planar while SO3 2- is pyramidal. Connects to sp2 hybridization, alkenes, resonance in nitrate and carbonate.
Visualization of Trigonal Planar Geometry
Carbon with three electron domains lies at the centre of a flat triangle with angles of about 120 degrees, as in formaldehyde. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In BF3 the F-B-F angles are all 120 degrees and the molecule is nonpolar.
Frequently asked questions about Trigonal Planar Geometry
What is Trigonal Planar Geometry?
Three groups at 120 degrees in a plane, as in BF3.
Is Trigonal Planar Geometry a beginner, intermediate or advanced topic?
Trigonal Planar Geometry is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Trigonal Planar Geometry?
Study these earlier topics in Bonding & Molecular Structure first: Electron Domain Geometry, Molecular Geometry, Linear Geometry.
What comes after Trigonal Planar Geometry?
The next topics in Bonding & Molecular Structure are Tetrahedral Geometry and Trigonal Bipyramidal Geometry.
Can I practise Trigonal Planar Geometry with quizzes?
Yes. The interactive lesson includes a self-check question on Trigonal Planar Geometry, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Trigonal Planar Geometry fits in Bonding & Molecular Structure
Trigonal Planar Geometry is topic 39 of 104 in Bonding & Molecular Structure.