Trigonal Bipyramidal Molecular Shape
A focused revision guide to the trigonal bipyramidal molecular shape, using PF5 as the key example. This page explains why five bonding pairs and no lone pairs around a central atom produce axial and equatorial positions with bond angles of 90°, 120° and 180°.
What Trigonal Bipyramidal Means
A trigonal bipyramidal molecule has five atoms bonded to a central atom and no lone pairs on that central atom. The five bonding regions repel and arrange themselves as far apart as possible.
The key example for this page is PF5. Phosphorus is the central atom and forms five P-F bonds to five fluorine atoms.
Key idea: five bonding pairs and no lone pairs around a central atom give a trigonal bipyramidal shape.
Why PF5 Is Trigonal Bipyramidal
In PF5, the central phosphorus atom is surrounded by five P-F bonding pairs. There are no lone pairs on the phosphorus atom, so the five bonding regions arrange to minimise bonding pair-bonding pair repulsion.
The shape has three equatorial positions in a trigonal plane and two axial positions above and below this plane. The molecular shape is therefore described as trigonal bipyramidal.
PF5 has five bonding regions around phosphorus. Three fluorine atoms occupy equatorial positions and two fluorine atoms occupy axial positions.
Trigonal Bipyramidal Shape of PF5
In phosphorus pentafluoride, phosphorus has five P-F bonding pairs and no lone pairs. Three fluorine atoms occupy the equatorial plane at 120° from each other, while two fluorine atoms occupy axial positions above and below the plane. This gives a trigonal bipyramidal shape.
Axial and Equatorial Positions
The trigonal bipyramidal shape has two different types of position around the central atom. The equatorial positions lie in the central triangular plane. The axial positions point above and below this plane.
This produces different bond angles within the same molecule. Equatorial bonds are 120° apart, axial bonds are 90° from equatorial bonds, and the two axial bonds are 180° apart.
Equatorial positions
Three positions in the central plane. The equatorial bond angles are 120°.
Axial positions
Two positions above and below the central plane. Each axial bond is 90° from the equatorial plane.
Opposite axial bonds
The two axial bonds are opposite each other, giving an axial-axial angle of 180°.
Trigonal Bipyramidal Examples
The trigonal bipyramidal shape is found when the central atom has five bonding pairs and no lone pairs. PF5 and PCl5 are common examples.
| Species | Central atom | Electron regions | Shape | Typical exam angles |
|---|---|---|---|---|
| PF5 | P | 5 bonding regions, 0 lone pairs | Trigonal bipyramidal | 90°, 120° and 180° |
| PCl5 | P | 5 bonding regions, 0 lone pairs | Trigonal bipyramidal | 90°, 120° and 180° |
How to Explain Trigonal Bipyramidal Shape in an Exam
A full exam explanation should connect the number of bonding pairs to electron-pair repulsion, then state the shape and the bond angles.
1. Identify the central atom
For PF5, the central atom is phosphorus.
2. Count bonding pairs and lone pairs
Phosphorus has five bonding pairs and no lone pairs around it.
3. Apply electron-pair repulsion
The five bonding pairs repel and arrange themselves as far apart as possible, giving three equatorial positions and two axial positions.
4. State the shape and angles
The molecule is trigonal bipyramidal with bond angles of 120° between equatorial bonds, 90° between axial and equatorial bonds, and 180° between the axial bonds.
Exam answer model: PF5 has five bonding pairs and no lone pairs around the central phosphorus atom. The bonding pairs repel and arrange themselves as far apart as possible. Three fluorine atoms occupy equatorial positions at 120° to each other, while two fluorine atoms occupy axial positions at 90° to the equatorial plane and 180° from each other. Therefore, PF5 is trigonal bipyramidal.
Common Exam Points
Do not use only one bond angle
Trigonal bipyramidal molecules have more than one relevant angle. State 120° for equatorial-equatorial, 90° for axial-equatorial and 180° for axial-axial when needed.
Separate trigonal planar from trigonal bipyramidal
Trigonal planar has three bonding regions and a 120° angle. Trigonal bipyramidal has five bonding regions with axial and equatorial positions.
Count around the central atom only
The shape is determined by bonding pairs and lone pairs around phosphorus in PF5, not by the total number of atoms in the formula alone.
Use electron-pair repulsion language
Explain that bonding pairs repel and arrange as far apart as possible. Since PF5 has no lone pairs on phosphorus, there is no lone pair compression to discuss.
Check Your Understanding
Use these short activities to check the trigonal bipyramidal shape, PF5, five bonding pairs, no lone pairs and the 90°, 120° and 180° bond angles.
QuickSnap
The trigonal bipyramidal shape is produced when a central atom has five bonding pairs and no lone pairs. PF5 has three equatorial bonds and two axial bonds, giving bond angles of 120°, 90° and 180°.
Memory line: 5 bonding pairs + 0 lone pairs = trigonal bipyramidal = 90°, 120° and 180°.
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The shapes of some species are being compared.
Which species is not tetrahedral?
Explain this difference in terms of structure and bonding.
FAQs
Common questions about trigonal bipyramidal molecular shape.
What is a trigonal bipyramidal molecular shape?
A trigonal bipyramidal molecular shape forms when a central atom has five bonding pairs and no lone pairs. Three atoms occupy equatorial positions and two atoms occupy axial positions.
Why is PF5 trigonal bipyramidal?
PF5 is trigonal bipyramidal because phosphorus has five P-F bonding pairs and no lone pairs around it. The five bonding regions repel and arrange themselves as far apart as possible.
What are the bond angles in PF5?
PF5 has 120° between equatorial bonds, 90° between axial and equatorial bonds, and 180° between the two axial bonds.
What is the difference between axial and equatorial positions?
Equatorial positions lie in the central triangular plane and are 120° apart. Axial positions are above and below this plane and are 90° from the equatorial positions.
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