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 a little smaller than the ideal 120°, 90° and 180°.
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.
Quick Check: Which Species Is SeeSaw-Shaped?
All four species have four bonded atoms, so count the electrons around each central atom to find the SeeSaw.
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.
A regular trigonal bipyramid has bond angles of 120° within the equatorial plane, 90° between an axial and an equatorial position and 180° between the two axial positions. A lone pair repels more strongly than a bonding pair, so in SF4 these angles close up to about 102°, about 87° and about 173°.
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.
How to count: start with the outer-shell electrons of the central atom. Add one electron for each negative charge and take one away for each positive charge. Each single bond uses one of those electrons and each double bond uses two, but a double or triple bond still counts as one bonding region. Divide the electrons left over by two to get the lone pairs. In SF4: sulfur has 6 outer-shell electrons, the molecule has no charge, and the four S-F single bonds use four of them. Two are left over, which is one lone pair. Four bonding regions and one lone pair give five regions, so the arrangement is trigonal bipyramidal and the shape is SeeSaw. For the ion ClF4+: chlorine has 7 outer-shell electrons, the positive charge takes one away to leave 6, the four bonds use four, and the two left over are again one lone pair.
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°.
Quick Check: Axial or Equatorial?
Drag the words into place to show why a lone pair prefers an equatorial position.
Seesaw Shape of SF4
Build sulfur tetrafluoride one layer at a time, from five electron pairs in a trigonal bipyramid to a seesaw, and see why the lone pair chooses an equatorial position.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
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.
Quick Check: Predict a New Molecule
Work through the prediction for tellurium tetrachloride one step at a time.
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 | Bond angles |
|---|---|---|---|---|
| SF4 | S | 4 bonding regions, 1 lone pair | SeeSaw | About 102°, 87° and 173° |
| SeF4 | Se | 4 bonding regions, 1 lone pair | SeeSaw | Slightly less than 120°, 90° and 180° |
| TeF4 | Te | 4 bonding regions, 1 lone pair | SeeSaw | Slightly less than 120°, 90° and 180° |
| IF4+ | I | 4 bonding regions, 1 lone pair | SeeSaw | Slightly less than 120°, 90° and 180° |
| ClF4+ | Cl | 4 bonding regions, 1 lone pair | SeeSaw | Slightly less than 120°, 90° and 180° |
Quick Check: A SeeSaw with Double Bonds?
Count the electrons used and the regions around xenon, then choose the correct description.
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. Start from the regular trigonal bipyramidal angles of 120°, 90° and 180°, then state that the equatorial lone pair pushes them in. In SF4 the equatorial F-S-F angle is about 102°, the axial to equatorial angles are about 87° and the axial to axial angle is about 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. The lone pair repels more strongly than the bonding pairs, so the angles are reduced from 120°, 90° and 180° to about 102°, about 87° and about 173°.
Quick Check: Whose Reasoning Is Right?
In each round, choose the one student statement that is fully correct.
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.
Give angles below the ideal values
Quote the trigonal bipyramidal angles of 120°, 90° and 180°, then say that the equatorial lone pair reduces them. For SF4, use about 102°, about 87° and about 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.
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. The lone pair repels more strongly than the bonding pairs, so the ideal angles of 120°, 90° and 180° are reduced to about 102°, 87° and 173° in SF4.
Memory line: 4 bonding pairs + 1 lone pair = SeeSaw = about 102°, 87° and 173°.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
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?
Start from the trigonal bipyramidal angles of 120°, 90° and 180°. The equatorial lone pair repels more strongly than the bonding pairs, so in SF4 the angles are reduced to about 102°, about 87° and about 173°.
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.
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