VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the 3D shape of a molecule by counting electron domains around the central atom and spacing them as far apart as possible. The electron domain geometry (all domains, including lone pairs) determines the framework; the molecular geometry (only the atoms you can see) gives the actual shape name. Lone pairs compress bond angles because they repel more strongly than bonding pairs.
Difficulty: Intermediate
Prerequisites: you need to be comfortable drawing Lewis structures and counting valence electrons (see the Ionic and Covalent Bonding study notes). If you cannot quickly identify lone pairs and bonding pairs in a Lewis structure, review that material first.
Molecular geometry is one of the most testable topics in general chemistry because it ties together several concepts: Lewis structures, electron counting, and three-dimensional thinking. The shape of a molecule determines its polarity, which in turn affects boiling points, solubility, and biological activity. This topic is the bridge between drawing flat Lewis structures on paper and understanding how molecules behave in three dimensions. If you are behind, note that every question on molecular shape starts with a Lewis structure, so that skill must come first.
VSEPR (Valence Shell Electron Pair Repulsion)
A model that predicts molecular geometry by assuming electron domains around a central atom arrange themselves to minimise repulsion. Think of it as: electrons hate being near each other, so they spread out as far as they can.
Electron domain (electron group)
Any region of electron density around the central atom. A single bond, a double bond, a triple bond, and a lone pair each count as one electron domain. In simple terms, when counting domains, every "cluster" of electrons (bonded or not) counts as one.
Electron domain geometry (electron geometry)
The arrangement of all electron domains around the central atom, including both bonding pairs and lone pairs. This is the underlying framework.
Molecular geometry (molecular shape)
The arrangement of only the atoms in a molecule (ignoring lone pairs). This is the shape you name on an exam. It can differ from the electron domain geometry when lone pairs are present.
Bond angle
The angle between two adjacent bonds on the central atom. Ideal angles come from the electron domain geometry, but lone pairs compress them.
Lone pair repulsion
Lone pairs occupy more space than bonding pairs because they are held closer to the central atom. The repulsion hierarchy is: lone pair to lone pair > lone pair to bonding pair > bonding pair to bonding pair.
Trigonal bipyramidal
The electron domain geometry for five domains around a central atom. It has two distinct positions: axial (top and bottom) and equatorial (around the middle). Lone pairs preferentially occupy equatorial positions.
Seesaw, T-shape, linear (5-domain family)
Molecular geometries that arise from a trigonal bipyramidal electron domain geometry when 1, 2, or 3 of the five domains are lone pairs, respectively.
Electron pairs (both bonding and lone) in the valence shell of the central atom repel each other.
They arrange themselves as far apart as possible to minimise that repulsion.
The resulting arrangement determines both the electron domain geometry and the molecular geometry.
A single bond = 1 domain.
A double bond = 1 domain (not 2).
A triple bond = 1 domain (not 3).
A lone pair = 1 domain.
Count all of these around the central atom to get the total number of electron domains.
2 domains → Linear (180°)
Example: BeCl₂. Two bonding domains, no lone pairs. Bond angle 180°.
3 domains → Trigonal planar (120°)
Example: BF₃. Three bonding domains, no lone pairs. Bond angle 120°.
4 domains → Tetrahedral (109.5°)
Example: CCl₄. Four bonding domains, no lone pairs. Bond angle 109.5°.
5 domains → Trigonal bipyramidal (90° and 120°)
Axial positions at 90° to equatorial, equatorial positions at 120° to each other.
6 domains → Octahedral (90°)
The molecular geometry describes only the positions of the atoms, not the lone pairs. When lone pairs are present, the shape name changes even though the electron domain geometry stays the same.
4 electron domains, tetrahedral framework:
4 bonding, 0 lone pairs → Tetrahedral (109.5°). Example: CCl₄.
3 bonding, 1 lone pair → Trigonal pyramidal (~107°). Example: NCl₃, NH₃.
2 bonding, 2 lone pairs → Bent / V-shaped (~104.5°). Example: H₂O, F₂O.
5 electron domains, trigonal bipyramidal framework:
5 bonding, 0 lone pairs → Trigonal bipyramidal. Example: PF₅.
4 bonding, 1 lone pair → Seesaw.
3 bonding, 2 lone pairs → T-shape.
2 bonding, 3 lone pairs → Linear.
Each lone pair on the central atom compresses adjacent bond angles by roughly 2.5°.
This is because lone pairs sit closer to the nucleus and spread out more than bonding pairs, taking up more angular space.
Repulsion strength: lone pair to lone pair > lone pair to bonding pair > bonding pair to bonding pair.
NCl₃: Nitrogen has 5 valence electrons. Three are used in bonds to chlorine, leaving one lone pair. Electron domain geometry: tetrahedral. Molecular geometry: trigonal pyramidal. Bond angle: approximately 107° (compressed from 109.5° by the lone pair).
SO₂: Sulfur has 6 valence electrons. It forms two double bonds to oxygen and retains one lone pair. Electron domain count: 3 (two double bonds + one lone pair). Electron domain geometry: trigonal planar. Molecular geometry: bent/angular. Bond angle: approximately 120° (slightly less due to the lone pair).
SeO₂: Selenium has 6 valence electrons (same group as sulfur). Same structure as SO₂. Electron domain geometry: trigonal planar. Molecular geometry: bent/angular.
AsH₃: Arsenic has 5 valence electrons. Three single bonds to hydrogen, one lone pair. Electron domain geometry: tetrahedral. Molecular geometry: trigonal pyramidal. Bond angle: approximately 107.5°.
F₂O: Oxygen has 6 valence electrons. Two single bonds to fluorine, two lone pairs. Electron domain geometry: tetrahedral. Molecular geometry: bent/V-shaped. Bond angle: approximately 104.5°.
NO₂⁻: Nitrogen has 5 valence electrons, plus 1 for the negative charge = 6. Two bonding domains to oxygen, one lone pair. Electron domain geometry: trigonal planar. Molecular geometry: angular/bent. Bond angle: less than 120°.
Molecular Shapes Reference (4 electron domains, tetrahedral family)
Bonded atoms | Lone pairs | Molecular geometry | Approx. bond angle |
|---|---|---|---|
4 | 0 | Tetrahedral | 109.5° |
3 | 1 | Trigonal pyramidal | ~107° |
2 | 2 | Bent / V-shaped | ~104.5° |
Molecular Shapes Reference (5 electron domains, trigonal bipyramidal family)
Bonded atoms | Lone pairs | Molecular geometry | Approx. bond angle |
|---|---|---|---|
5 | 0 | Trigonal bipyramidal | 90° / 120° |
4 | 1 | Seesaw | <90° / <120° |
3 | 2 | T-shape | <90° |
2 | 3 | Linear | 180° |
Lone pair compression rule of thumb:
Each lone pair subtracts roughly 2.5° from the ideal bond angle of the electron domain geometry.
The shape of water (bent, not linear) is the reason it is a polar molecule, which is why it dissolves salts, sustains life, and has an unusually high boiling point for its molecular weight. Drug design relies heavily on molecular geometry: a drug molecule must fit into an enzyme's active site like a key in a lock, and that fit depends on bond angles and 3D shape. Atmospheric chemistry, too, depends on molecular shape, as the bent geometry of SO₂ and the linear geometry of CO₂ affect how these molecules absorb infrared radiation (the greenhouse effect).
Students often confuse electron domain geometry with molecular geometry. The electron domain geometry includes lone pairs; the molecular geometry does not. They are only the same when there are zero lone pairs on the central atom.
A double or triple bond counts as one electron domain, not two or three. This is one of the most common mistakes on exams.
Students sometimes think lone pairs do not affect molecular shape. They do: lone pairs change the shape name and compress bond angles.
Some students assume that "trigonal planar" and "bent" are unrelated. Bent is what you get when you start with a trigonal planar electron domain geometry and one of the three domains is a lone pair (as in SO₂). Similarly, bent from a tetrahedral framework (as in H₂O) is a different angle from bent from a trigonal planar framework.
⚠️ Expect a question asking you to distinguish electron domain geometry from molecular geometry for the same molecule. This is a near-certain exam topic.
⚠️ Know the tetrahedral family cold: tetrahedral (4 bonds, 0 lone pairs), trigonal pyramidal (3 bonds, 1 LP), bent (2 bonds, 2 LP), and the approximate bond angles for each.
⚠️ Be able to explain why bond angles in NH₃ (~107°) and H₂O (~104.5°) are less than the ideal tetrahedral angle of 109.5°.
⚠️ Double and triple bonds counting as one domain is a favourite trick question.
⚠️ The 5-domain family (trigonal bipyramidal → seesaw → T-shape → linear) appears in more advanced questions. Know the pattern even if the worked examples focus on the 4-domain family.
True or False: A double bond counts as two electron domains.
Fill in the blank: A molecule with 4 electron domains and 1 lone pair has a ______ molecular geometry.
True or False: Electron domain geometry and molecular geometry are always the same.
Fill in the blank: The repulsion order from greatest to least is: lone pair–lone pair > ______ > bonding pair–bonding pair.
True or False: H₂O has a tetrahedral electron domain geometry but a bent molecular geometry.
Q: What is the electron domain geometry and molecular geometry of NCl₃?
A: Nitrogen has 3 bonding domains (to Cl) and 1 lone pair, for 4 total electron domains. Electron domain geometry: tetrahedral. Molecular geometry: trigonal pyramidal. Bond angle: approximately 107°.
Q: Why is the bond angle in water (~104.5°) smaller than the ideal tetrahedral angle (109.5°)?
A: Water has two lone pairs on oxygen. Lone pairs repel more strongly than bonding pairs, compressing the H–O–H bond angle. Each lone pair reduces the angle by roughly 2.5° from the ideal.
Q: Predict the molecular geometry of SeO₂.
A: Selenium has 6 valence electrons. It forms two double bonds to oxygen and has one lone pair. Total electron domains: 3. Electron domain geometry: trigonal planar. Molecular geometry: bent/angular. Bond angle: slightly less than 120°.
Q: A molecule has 5 electron domains, 4 of which are bonding pairs. What is its molecular geometry?
A: Seesaw. The electron domain geometry is trigonal bipyramidal. With one lone pair (which occupies an equatorial position), the molecular geometry is seesaw.
Q: Explain the difference between electron domain geometry and molecular geometry.
A: Electron domain geometry accounts for all electron domains (bonding pairs and lone pairs) and describes the arrangement of all electron density around the central atom. Molecular geometry accounts for only the atoms (ignoring lone pairs) and describes the actual shape of the molecule. They differ whenever lone pairs are present on the central atom.
Molecular geometry connects directly to molecular polarity (a bent molecule like H₂O is polar; a symmetric molecule like CO₂ is not), which leads into intermolecular forces (dipole-dipole, hydrogen bonding, London dispersion). Those forces, in turn, explain macroscopic properties such as boiling points and solubility. In organic chemistry, VSEPR provides the foundation for understanding hybridisation (sp, sp², sp³), which is how chemists describe the same geometries using orbital language.
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