SeeSaw Molecular Shape
A focused revision guide to the SeeSaw molecular shape, using SF4 as the key example. This page explains why four bonding pairs and one lone pair around a central atom give a SeeSaw arrangement with bond angles close to 119° and 89° in simplified exam tables.
What SeeSaw Means
A SeeSaw molecule has four atoms bonded to a central atom, with the bonded atoms arranged like a tilted seesaw. The shape forms when there are four bonding pairs and one lone pair around the central atom.
The key example for this page is SF4. Sulfur is the central atom, four fluorine atoms are bonded to sulfur, and sulfur also has one lone pair.
Key idea: four bonding pairs and one lone pair around a central atom give a SeeSaw shape when the lone pair occupies an equatorial position in a trigonal bipyramidal electron-pair arrangement.
Why SF4 Is SeeSaw-Shaped
In SF4, the central sulfur atom is surrounded by four S-F bonding pairs and one lone pair. This gives five regions of negative charge around sulfur.
Five electron regions arrange themselves as a trigonal bipyramidal electron-pair arrangement to minimise repulsion. The lone pair occupies an equatorial position because this gives fewer 90° lone-pair interactions than placing it in an axial position.
The molecular shape only describes the positions of the atoms. Therefore, SF4 is described as SeeSaw-shaped, even though its electron-pair arrangement is trigonal bipyramidal.
A SeeSaw species has four bonding regions and one lone pair around the central atom. SF4 is the standard example, with simplified exam-table angles close to 119° and 89°.
Seesaw Shape of SF4
In sulfur tetrafluoride, sulfur has four S-F bonding pairs and one lone pair. The five electron regions arrange themselves as a trigonal bipyramidal electron-pair arrangement. The lone pair occupies an equatorial position, leaving two axial bonds and two equatorial bonds to form a seesaw-shaped molecule.
Electron-Pair Arrangement vs Molecular Shape
The electron-pair arrangement in SF4 is based on a trigonal bipyramidal arrangement because there are five electron regions around sulfur.
The molecular shape is different. Lone pairs are not counted as atoms when naming the shape, so the visible arrangement of the four fluorine atoms is SeeSaw-shaped.
Five electron regions
Four bonding pairs and one lone pair arrange themselves as far apart as possible around sulfur.
Equatorial lone pair
The lone pair occupies an equatorial position in the trigonal bipyramidal arrangement. This reduces the number of close 90° lone-pair interactions.
SeeSaw bonded atoms
The remaining four bonding pairs form two axial bonds and two equatorial bonds, producing a SeeSaw shape.
SeeSaw Shape Examples
The SeeSaw shape is found when the central atom has four bonding pairs and one lone pair. The common exam examples include sulfur, selenium, tellurium and iodine or chlorine species with five electron regions around the central atom.
| Species | Central atom | Electron regions | Shape | Typical exam angles |
|---|---|---|---|---|
| SF4 | S | 4 bonding regions, 1 lone pair | SeeSaw | 119° and 89° |
| SeF4 | Se | 4 bonding regions, 1 lone pair | SeeSaw | 119° and 89° |
| TeF4 | Te | 4 bonding regions, 1 lone pair | SeeSaw | 119° and 89° |
| IF4+ | I | 4 bonding regions, 1 lone pair | SeeSaw | 119° and 89° |
| ClF4+ | Cl | 4 bonding regions, 1 lone pair | SeeSaw | 119° and 89° |
How to Explain SeeSaw Shape in an Exam
A full exam explanation should connect the number of electron regions to electron-pair repulsion, then explain why the lone pair is equatorial and why the bonded atoms form a SeeSaw shape.
1. Identify the central atom
For SF4, the central atom is sulfur.
2. Count bonding pairs and lone pairs
Sulfur has four bonding pairs and one lone pair around it.
3. Apply electron-pair repulsion
The five electron regions arrange themselves as far apart as possible. The lone pair occupies an equatorial position to reduce repulsion.
4. State the shape and angles
The four bonded atoms form a SeeSaw shape. In simplified A Level exam tables, the bond angles are close to 119° and 89°. For SF4, the equatorial F-S-F angle is commonly described in more detailed chemistry as close to 102°, with axial-equatorial angles close to 87° and the axial-axial angle close to 173°.
Exam answer model: SF4 has four bonding pairs and one lone pair around the central sulfur atom. The five electron regions arrange themselves as a trigonal bipyramidal electron-pair arrangement. The lone pair occupies an equatorial position to minimise repulsion. Therefore, SF4 is SeeSaw-shaped, with bond angles close to 119° and 89° in the simplified exam table.
Common Exam Points
Do not call SF4 trigonal bipyramidal
The electron-pair arrangement is trigonal bipyramidal, but the molecular shape is SeeSaw because the lone pair is not counted as an atom.
State the lone pair is equatorial
The lone pair occupies an equatorial position. This reduces repulsion compared with placing the lone pair in an axial position.
Use the correct angle detail for the context
For simple exam-table recall, use approximately 119° and 89°. For detailed SF4 geometry, the bp-bp angles include about 87°, about 102° and close to 173°.
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 atoms in the species.
Check Your Understanding
Use these short activities to check the SeeSaw shape, SF4, four bonding pairs, one lone pair and the approximate bond angles.
QuickSnap
The SeeSaw shape is produced when a central atom has four bonding pairs and one lone pair. The lone pair occupies an equatorial position in a trigonal bipyramidal electron-pair arrangement, leaving four bonded atoms in a SeeSaw arrangement with simplified exam-table bond angles close to 119° and 89°.
Memory line: 4 bonding pairs + 1 lone pair = SeeSaw = about 119° and 89°.
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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.
SeeSaw FAQs
These questions target the points students most often lose marks on when explaining the SeeSaw molecular shape.
Why is SF4 SeeSaw-shaped?
SF4 is SeeSaw-shaped because sulfur has four bonding pairs and one lone pair. The five electron regions arrange as trigonal bipyramidal, but the lone pair occupies an equatorial position, leaving two axial S-F bonds and two equatorial S-F bonds.
What are the bond angles in a SeeSaw molecule?
In simplified A Level exam tables, the angles are often given as about 119° and 89°. In SF4, more detailed values are approximately 87°, 102° and 173° because the equatorial lone pair compresses the bond-pair angles.
Is SeeSaw the same as trigonal bipyramidal?
No. Trigonal bipyramidal is the electron-pair arrangement for five regions of negative charge. SeeSaw is the molecular shape after the single lone pair is ignored when naming the shape.
Why does the lone pair in SF4 occupy an equatorial position?
The lone pair occupies an equatorial position because this minimises repulsion. An equatorial lone pair has fewer 90° interactions with bonding pairs than an axial lone pair.
Copyright notice: This OLS revision page was written and designed by Dr. Mohammed Al-Fatah for Online Learning System. All written explanations, page structure, visual learning design and embedded teaching content are protected by copyright. Do not copy, reproduce or redistribute this material without permission.