Trigonal Pyramidal Molecular Shape
A focused revision guide to the trigonal pyramidal molecular shape, using NH3 as the key example. This page explains why three bonding pairs and one lone pair around a central atom give a three-dimensional pyramidal arrangement with bond angles of about 107°.
- 2A.11-d
- 2A.12-a
- 2A.12-b
What these spec points say
- 2A.11-d understand reasons for the shape of, and bond angles in, NH3
- 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 Pyramidal Shape of NH₃
Choose a step to watch four electron pairs form a tetrahedron, one stay as a lone pair and squeeze the bond angles to 107°.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Forces Between Charges
Before you start, check the GCSE ideas about forces between charges that this page builds on.
What Trigonal Pyramidal Means
A trigonal pyramidal molecule has three atoms bonded to a central atom, with one lone pair also present on the central atom. The three bonded atoms form a triangular base, while the central atom sits above the base to give a pyramidal shape.
The key example for this page is NH3. Nitrogen is the central atom, each hydrogen atom is bonded to nitrogen, and nitrogen also has one lone pair.
Key idea: three bonding pairs and one lone pair around a central atom give a trigonal pyramidal shape with bond angles of about 107°.
Why NH3 Is Trigonal Pyramidal
In NH3, the central nitrogen atom is surrounded by three N-H bonding pairs and one lone pair. These four regions of negative charge repel one another and are based on a tetrahedral electron-pair arrangement.
The molecular shape only considers the positions of the atoms, not the invisible lone pair. Therefore, NH3 is described as trigonal pyramidal, not tetrahedral.
A trigonal pyramidal species has three bonding regions and one lone pair around the central atom, giving a bond angle of about 107°.
Nitrogen keeps 2 of its 5 outer electrons as a lone pair: three bonding regions and one lone pair give a trigonal pyramid.
Quick Check: Arrangement or Shape?
Choose the line that gives the arrangement and the shape of SbCl₃.
Watch: one lone pair squeezes the bonds
Swap one bonding pair for a lone pair and the model squeezes together. How far does the angle fall?
Bonding Pairs, Lone Pairs and Bond Angle
The electron-pair arrangement around nitrogen in NH3 is based on four electron domains: three bonding pairs and one lone pair. A lone pair occupies more space around the central atom than a bonding pair because it is attracted to only one nucleus.
This means the lone pair repels the N-H bonding pairs more strongly than the bonding pairs repel each other. The H-N-H bond angle is compressed from the tetrahedral value of 109.5° to about 107°.
| Feature | Trigonal pyramidal value | Meaning in NH3 |
|---|---|---|
| Number of bonding pairs | 3 | Three N-H bonding pairs form the visible triangular base of the pyramid. |
| Number of lone pairs | 1 | The lone pair repels more strongly than bonding pairs and compresses the bond angle. |
| Bond angle | about 107° | The angle is smaller than 109.5° because of lone-pair repulsion. |
| Molecular shape | Trigonal pyramidal | Only the positions of the bonded atoms are used to name the molecular shape. |
Swapping a bonding pair for a lone pair closes the angle by about 2.5° each time: 109.5°, 107°, 104.5°.
Quick Check: Predict a New Ion
Drag the words into place to predict the shape of the methyl anion.
Examples of Trigonal Pyramidal Species
Several molecules and ions use the same three-bonding-pair and one-lone-pair arrangement around the central atom. Other examples are NCl3, PF3, ClO3– and H3O+.
For exam purposes, focus on the central atom, count the bonding pairs and lone pairs around it, then state the shape and approximate bond angle.
| Species | Central atom | Electron regions around central atom | Shape | Bond angle |
|---|---|---|---|---|
| NH3 | N | 3 bonding regions, 1 lone pair | Trigonal pyramidal | about 107° |
| NCl3 | N | 3 bonding regions, 1 lone pair | Trigonal pyramidal | about 107° |
| PF3 | P | 3 bonding regions, 1 lone pair | Trigonal pyramidal | about 107° |
| ClO3– | Cl | 3 bonding regions, 1 lone pair | Trigonal pyramidal | about 107° |
| H3O+ | O | 3 bonding regions, 1 lone pair | Trigonal pyramidal | about 107° |
Quick Check: Pyramid or Not?
Work out each species on paper, then flip the card to check.
How to Explain Trigonal Pyramidal Shape in an Exam
A full exam explanation should connect the number of electron pairs to electron-pair repulsion, then explain why the bond angle is slightly smaller than the tetrahedral angle.
1. Identify the central atom
For NH3, the central atom is nitrogen.
2. Count bonding pairs and lone pairs
Nitrogen has three bonding pairs and one lone pair around it.
3. Apply electron-pair repulsion
The electron pairs repel and arrange themselves as far apart as possible, but the lone pair repels more strongly than a bonding pair.
4. State the shape and angle
The molecule is trigonal pyramidal with bond angles of about 107°.
Exam answer model: NH3 has three bonding pairs and one lone pair around the central nitrogen atom. The electron pairs repel and arrange themselves as far apart as possible.
The lone pair repels the bonding pairs more strongly, compressing the H-N-H bond angle from 109.5° to about 107°. Therefore, NH3 is trigonal pyramidal.
Quick Check: Explain the Difference
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 pyramidal questions often test whether you can distinguish the electron-pair arrangement from the molecular shape.
Do not call NH3 tetrahedral
The electron-pair arrangement is based on a tetrahedron, but the molecular shape is trigonal pyramidal because one region is a lone pair rather than a bonded atom.
Do not write 109.5°
109.5° is the ideal tetrahedral angle. In NH3, the lone pair compresses the H-N-H angle to about 107°.
Use lone-pair repulsion language
Marks usually require the idea that a lone pair repels bonding pairs more strongly than bonding pairs repel each other.
Count around the central atom only
The shape is determined by the bonding pairs and lone pairs around the central atom, not by the total number of bonds or lone pairs in the whole species.
QuickSnap
The trigonal pyramidal shape is produced when a central atom has three bonding pairs and one lone pair. The lone pair repels bonding pairs more strongly and compresses the bond angle to about 107°.
Memory line: 3 bonding pairs + 1 lone pair = trigonal pyramidal = about 107°.
FAQs
These questions target the common points students confuse when learning the trigonal pyramidal molecular shape.
What makes a molecule trigonal pyramidal?
A molecule is trigonal pyramidal when the central atom has three bonding pairs and one lone pair. The bonded atoms form a pyramidal shape because the lone pair occupies one electron domain but is not shown as an atom in the molecular shape.
Why is NH3 trigonal pyramidal?
NH3 has three N-H bonding pairs and one lone pair around the central nitrogen atom. The four electron domains are based on a tetrahedral arrangement, but the visible molecular shape is trigonal pyramidal.
What is the bond angle in NH3?
The H-N-H bond angle in NH3 is about 107°. It is smaller than 109.5° because the lone pair repels the bonding pairs more strongly.
Why is NH3 not called tetrahedral?
The electron-pair arrangement around nitrogen is based on a tetrahedron, but the molecular shape is named from the positions of the atoms only. Since one of the four electron domains is a lone pair, the molecular shape is trigonal pyramidal.
Why does a lone pair repel more strongly than a bonding pair?
A lone pair is held by only one nucleus, so its electron density sits closer to the central atom and spreads more widely. It pushes the bonding pairs together, which is why NH3 is 107° and not 109.5°.
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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