Trigonal Planar Molecular Shape
A focused revision guide to the trigonal planar molecular shape, using BCl3 as the key example. This page explains why three bonding pairs and no lone pairs around a central atom give 120° bond angles in one flat plane.
- 2A.10-a
- 2A.11-b
- 2A.12-a
- 2A.12-b
What these spec points say
- 2A.10-a understand that the shape of a simple molecule is determined by repulsion between electron pairs around a central atom
- 2A.11-b understand reasons for the shape of, and bond angles in, BCl3
- 2A.12-a be able to predict the shapes of simple molecules analogous to specified examples using electron-pair repulsion theory
- 2A.12-b be able to predict the bond angles in simple molecules analogous to specified examples using electron-pair repulsion theory
Trigonal Planar Shape of BCl₃
Choose a step to watch three electron pairs repel into a flat triangle, bond to chlorine, read the 120° angles and turn edge-on to see the molecule is flat.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Double Covalent Bonds
Before you start, check the GCSE ideas about double bonds that this page builds on.
What Trigonal Planar Means
A trigonal planar molecule has three atoms arranged around a central atom in a flat triangular pattern. The word trigonal means three positions around the central atom, and planar means all the atoms lie in the same plane.
The key example for this page is BCl3. Boron is the central atom and each chlorine atom is attached to boron by a covalent bond.
Key idea: three bonding regions and no lone pairs around a central atom give a trigonal planar shape with 120° bond angles.
Watch: three bonding pairs, as far apart as possible
Three bonding pairs around one central atom spread out to 120 degrees, all in one plane. Why, and what is the shape called?
Why BCl3 Is Trigonal Planar
In BCl3, the central boron atom is surrounded by three B-Cl bonding pairs. These bonding pairs are regions of negative charge, so they repel each other and move as far apart as possible.
With three equal bonding regions and no lone pairs, the maximum separation is achieved when the bonds are spread evenly in one plane. This places each B-Cl bond 120° from the next.
Boron has 3 outer electrons, all used in bonds: three regions and no lone pairs give a trigonal planar molecule.
Quick Check: Why Flat?
Choose the statement that explains why borane is flat.
BCl3 has three B-Cl bonds around boron and no lone pairs on the central atom, so the three bonds spread out evenly.
Bonding Pairs, Lone Pairs and Bond Angle
The shape is decided by the number of electron domains around the central atom. In a simple shape question, each single bond counts as one bonding region. BCl3 has three bonding regions around boron and no lone pair regions on boron.
| Feature | Trigonal planar value | Meaning in BCl3 |
|---|---|---|
| Number of bonding pairs | 3 | Three B-Cl bonding pairs repel each other equally. |
| Number of lone pairs | 0 | No lone pair compression occurs, so the ideal angle is maintained. |
| Bond angle | 120° | The three bonds are evenly spaced in a flat triangular arrangement. |
| Molecular shape | Trigonal planar | All four atoms lie in one plane. |
Two regions get as far apart as possible at 180°, three regions at 120° in one plane.
Quick Check: Count the Regions
Count the bonding regions and lone pairs, then type the numbers.
Examples of Trigonal Planar Species
Several molecules and ions use the same three-region arrangement around the central atom. Other examples are BF3, AlCl3, SO3, NO3– and CO32-.
For exam purposes, focus on the central atom, count the electron domains around it, then state the shape and bond angle.
| Species | Central atom | Electron regions around central atom | Shape | Bond angle |
|---|---|---|---|---|
| BF3 | B | 3 bonding regions, 0 lone pairs | Trigonal planar | 120° |
| AlCl3 | Al | 3 bonding regions, 0 lone pairs | Trigonal planar | 120° |
| SO3 | S | 3 bonding regions, 0 lone pairs | Trigonal planar | 120° |
| NO3– | N | 3 bonding regions, 0 lone pairs | Trigonal planar | 120° |
| CO32- | C | 3 bonding regions, 0 lone pairs | Trigonal planar | 120° |
Quick Check: Trigonal Planar or Not?
Decide quickly whether each species is trigonal planar.
How to Explain Trigonal Planar Shape in an Exam
A full exam explanation should name the electron pairs around the central atom and connect this directly to repulsion and angle.
1. Identify the central atom
For BCl3, the central atom is boron.
2. Count bonding pairs and lone pairs
Boron has three bonding pairs and no lone pairs around it.
3. Apply electron-pair repulsion
The three bonding pairs repel equally and move as far apart as possible.
4. State the shape and angle
The molecule is trigonal planar with bond angles of 120°.
Exam answer model: BCl3 has three bonding pairs and no lone pairs around the central boron atom. The bonding pairs repel equally and arrange themselves as far apart as possible in one plane. Therefore, BCl3 is trigonal planar with bond angles of 120°.
Quick Check: Write the Exam Answer
Write a short explanation, then compare it with the mark points and the model answer.
Quick Check: The Whole Page in Four Statements
In each round, pick the one accurate statement to review everything on this page.
Common Exam Points
Trigonal planar questions often test whether you can distinguish between the number of atoms attached to the central atom and the number of electron domains around the central atom.
Do not write 109.5°
109.5° is the tetrahedral angle. Trigonal planar species have three electron regions and an ideal angle of 120°.
Do not add a lone pair to boron in BCl3
For this shape model, boron has three bonding pairs and no lone pairs around the central atom.
State “in one plane” when useful
The planar part of trigonal planar means the atoms lie in a flat arrangement.
Use electron-pair repulsion language
Marks usually require the idea that bonding pairs repel and arrange themselves as far apart as possible.
QuickSnap
The trigonal planar shape is produced when a central atom has three bonding pairs and no lone pairs. The bonding pairs repel equally and spread out evenly in one plane, giving 120° bond angles.
Memory line: 3 bonding pairs + 0 lone pairs = trigonal planar = 120°.
FAQs
These questions target the common points students confuse when learning the trigonal planar molecular shape.
What makes a molecule trigonal planar?
A molecule is trigonal planar when the central atom has three bonding regions and no lone pairs. The bonding pairs repel equally and arrange themselves 120° apart in one plane.
Why is BCl3 trigonal planar?
BCl3 has three B-Cl bonding pairs around the central boron atom and no lone pairs on boron in the shape model. The three bonding pairs repel equally, so the molecule is trigonal planar with 120° bond angles.
What is the bond angle in a trigonal planar molecule?
The ideal bond angle in a trigonal planar molecule is 120°.
Are all atoms in a trigonal planar molecule in one plane?
Yes. In a trigonal planar molecule such as BCl3, the central atom and the three surrounding atoms lie in the same plane.
Why is NH3 not trigonal planar when BF3 is?
BF3 has three bonding pairs and no lone pairs, so it is trigonal planar at 120°. NH3 also has a lone pair, giving four regions, so its shape is trigonal pyramidal at about 107°.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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