9.1 VSEPR Theory and Molecular Geometry
PRE-HEALTH PREP
VSEPR Theory & Molecular Shapes
General Chemistry 9.1
Lesson Overview
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts three-dimensional molecular shapes by counting electron domains — bonding pairs and lone pairs — around a central atom and determining how they arrange to minimize repulsion. The 5 electron domain geometries (linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral) give rise to 12 distinct molecular geometries, from simple linear molecules to square planar arrangements, depending on how many domains are lone pairs versus bonding pairs. Bond angles for each geometry are presented alongside the systematic 6-step workflow for converting a Lewis structure into a complete molecular geometry prediction.
Table of Contents
- What Is VSEPR Theory?
- What Are Electron Domains?
- The 5 Electron Domain Geometries
- Bond Angles Reference Chart
- Electron Domain Geometry vs. Molecular Geometry
- Molecular Shapes: 2 Electron Domains
- Molecular Shapes: 3 Electron Domains
- Molecular Shapes: 4 Electron Domains
- Molecular Shapes: 5 Electron Domains
- Molecular Shapes: 6 Electron Domains
- 6-Step Method: Lewis Structure to Molecular Geometry
- Key Takeaways
What Is VSEPR Theory?
VSEPR theory (Valence Shell Electron Pair Repulsion) is the model chemists use to predict the three-dimensional molecular geometry of molecules from their Lewis structures. It rests on a single core idea: electron domains around a central atom repel one another and spread as far apart as possible to minimize that repulsion, lowering the molecule's energy and increasing its stability. Because molecular geometry controls properties like polarity, reactivity, and biological activity, understanding VSEPR theory is foundational for all of general chemistry.
Molecular geometry — the three-dimensional shape of a molecule — is one of the most visually satisfying topics in general chemistry. To determine molecular shape, you need to start with VSEPR theory, which provides the conceptual foundation for predicting every molecular geometry you will encounter.
Why "VSEPR" and not "VSEPRT"? Technically the full name ends with "repulsion theory," but adding the T makes the acronym unpronounceable. So we drop it. The official acronym is VSEPR (often pronounced "vesper"), and that is the name you will see on every exam.
What Are Electron Domains?
To apply VSEPR theory and predict molecular geometry, you must count the electron domains around a central atom. An electron domain is any region of electron density — and there are exactly 2 types:
- Bonding domain: a bond to another atom — single, double, or triple. The bond order does not matter; a triple bond still counts as one electron domain because it still points in one direction toward one atom.
- Non-bonding domain (lone pair): a pair of electrons on the central atom that is not shared with any other atom.
Count the bonding domains and the lone pairs together to get the total number of electron domains. That total number determines the electron domain geometry (EDG), which describes how all the electron groups — bonding and non-bonding — are arranged in 3D space.
Key rule: A double bond and a triple bond each count as one electron domain, not two or three. It is the number of atoms the central atom is bonded to, not the number of bond lines drawn in the Lewis structure, that sets the domain count.
In this lesson you are responsible for understanding the molecular geometries associated with 2 through 6 electron domains — 5 distinct electron domain geometries in total.
The 5 Electron Domain Geometries
2 Electron Domains: Linear Geometry
With only 2 electron domains, maximum separation is achieved by placing them on exactly opposite sides of the central atom, 180° apart. Both the electron domain geometry and the molecular geometry are linear. Two electron domains is the only case where the two geometries are guaranteed to share the same name regardless of bonding versus lone-pair status — because there is no lone-pair variant to consider at this count.
3 Electron Domains: Trigonal Planar Geometry
Three electron domains spread out in a single plane. Dividing 360° evenly among 3 groups gives 120° bond angles, with the three domains pointing toward the corners of an equilateral triangle. Because this flat, triangular arrangement exists entirely within one plane, the electron domain geometry is called trigonal planar.
4 Electron Domains: Tetrahedral Geometry
Four electron domains cannot achieve maximum separation by staying flat. A planar arrangement would give bond angles of 90°; by expanding into three-dimensional space, the domains can push apart to 109.5°. The resulting shape — three domains forming the base of a pyramid, one pointing straight up — is called a tetrahedron, so the geometry is tetrahedral.
Common mistake with tetrahedral structures: Lewis structures look flat, which tempts students to read bond angles as 90° or 180°. In a tetrahedral molecule, every pair of bonds is 109.5° apart — no exceptions. There are no 90° or 180° angles in a perfect tetrahedron.
To represent 3D molecular geometry on a two-dimensional page, chemists use two conventions: a wedge bond (solid, filled wedge) indicates a bond coming toward you out of the plane, while a dashed bond indicates a bond going away from you into the plane.
5 Electron Domains: Trigonal Bipyramidal Geometry
Five electron domains require an expanded octet on the central atom, which is only possible for elements in the third period and beyond (e.g., phosphorus, sulfur). Three of the five domains occupy a horizontal triangular plane (the equatorial positions), while the remaining two point straight up and straight down (the axial positions). This arrangement creates two triangle-based pyramids fused base-to-base — one right-side up, one inverted — giving the name trigonal bipyramidal.
6 Electron Domains: Octahedral Geometry
Six electron domains also require an expanded octet. Four domains lie in a horizontal square plane, with one domain directly above and one directly below. The resulting shape has eight triangular faces (four on the upper pyramid, four on the lower), which is why this geometry is called octahedral (from the Greek okto, eight).
Do not let "octa" mislead you: octahedral refers to eight faces, not eight electron domains. An octahedral molecule has exactly six electron domains.
Bond Angles Reference Chart
Each electron domain geometry has characteristic bond angles. Memorizing these bond angles is essential for exam success in general chemistry — especially the 109.5° tetrahedral angle, which is among the most frequently tested values. The expanded-octet geometries have more than one set of angles because not all positions are equivalent.
| Electron Domains | Electron Domain Geometry | Bond Angle(s) |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal Planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal Bipyramidal | Equatorial–equatorial: 120° Equatorial–axial: 90° Axial–axial: 180° |
| 6 | Octahedral | Adjacent: 90° Opposite: 180° |
Memorization priority: The 109.5° tetrahedral bond angle is tested extremely frequently. Make sure that one is locked in. The trigonal bipyramidal set of 90°/120°/180° is the trickiest because there are three distinct values to keep track of.
| Electron Domains | Electron Domain Geometry | Bond Angles | Hybridization of Central Atom |
|---|---|---|---|
| 2 | Linear | 180° | sp |
| 3 | Trigonal Planar | 120° | sp2 |
| 4 | Tetrahedral | 109.5° | sp3 |
| 5 | Trigonal Bipyramidal | 90°, 120°, 180° | sp3d |
| 6 | Octahedral | 90°, 180° | sp3d2 |
Electron Domain Geometry vs. Molecular Geometry
Once you know the electron domain geometry, you are ready to determine the molecular geometry — the name given specifically to the arrangement of atoms, ignoring where the lone pairs sit. This distinction between electron domain geometry and molecular geometry is one of the most important concepts in VSEPR theory.
Here is the key distinction:
- Electron domain geometry (EDG) is based on the total number of electron domains, both bonding and non-bonding.
- Molecular geometry (MG) is based only on the positions of atoms. Lone pairs are invisible in the molecular geometry name.
When all electron domains are bonding, the molecular geometry name is identical to the electron domain geometry name. As soon as any domain is replaced by a lone pair, the arrangement of atoms changes shape — and gets a new molecular geometry name. The electron domain geometry, however, never changes; it is always determined by the total domain count.
Example: The EDG stays, the molecular geometry changes
CH₄, NH₃, and H₂O all have 4 electron domains and therefore all share the same EDG: tetrahedral. But their molecular geometries differ — tetrahedral, trigonal pyramidal, and bent, respectively — because the number of lone pairs (0, 1, and 2) changes the atom-only shape.
Molecular Shapes: 2 Electron Domains
With 2 electron domains, there is only one possible molecular geometry: linear. Since there are no lone pairs to introduce, the molecular geometry is always the same as the electron domain geometry. The bond angle is always 180°.
Example: Linear molecular geometry in 3 molecules
All three structures below have 2 electron domains on the central atom:
- BeCl₂ — beryllium bonded to two chlorine atoms via single bonds, no lone pairs. Two electron domains.
- HCN — carbon bonded to hydrogen (single bond) and nitrogen (triple bond). The triple bond still counts as one domain. Two electron domains, linear.
- CO₂ — carbon bonded to two oxygen atoms via double bonds. Each double bond is one domain. Two electron domains, linear.
The lesson: single bonds, double bonds, and triple bonds all count as one electron domain each. Only the number of bonded atoms (and lone pairs) matters.
Molecular Shapes: 3 Electron Domains
Three electron domains give a trigonal planar electron domain geometry. The molecular geometry depends on whether all 3 domains are bonding or one is a lone pair, producing 2 possible molecular shapes.
Trigonal Planar Molecular Geometry
When all 3 electron domains are bonding atoms, the molecular geometry is trigonal planar, identical to the EDG. All bond angles are 120°.
Example: BF₃ and carbonyl species
Boron in BF₃ is bonded to three fluorine atoms with no lone pairs — three bonding domains, trigonal planar molecular geometry. Similarly, a carbon double-bonded to one atom and single-bonded to two others (e.g., the carbonyl carbon in formaldehyde, CH₂O) has three bonding domains and trigonal planar geometry.
Bent Molecular Geometry (from Trigonal Planar EDG)
When one of the 3 domains is a lone pair, the molecular geometry is bent. With only two bonded atoms and one central atom, the molecule forms a three-atom, non-linear arrangement. The bond angle is slightly less than 120° due to the lone pair's greater repulsion.
Example: SO₂
Sulfur in SO₂ is bonded to two oxygen atoms and carries one lone pair — three electron domains total, so EDG is trigonal planar. The molecular geometry, however, is bent, because the two oxygen atoms and the central sulfur are not in a straight line.
The actual bond angle in SO₂ is slightly less than 120° (approximately 118°). The lone pair exerts greater repulsion than a bonding pair, compressing the O–S–O angle. Additionally, the double-bond domain carries more electron density than a single bond, which also perturbs the ideal 120° angles slightly.
Lone pairs and double bonds both compress bond angles: Lone pairs repel more strongly than bonding pairs because their electrons are held closer to the central atom and spread over a larger volume. Double (and triple) bonds also carry more electron density than single bonds, increasing repulsion in that domain. Both effects push the remaining bond angles below the ideal value — usually by just a few degrees.
Molecular Shapes: 4 Electron Domains
Four electron domains give a tetrahedral electron domain geometry. This is the most important set of molecular geometries in general chemistry because it produces 3 distinct molecular shapes depending on how many lone pairs are present.
Tetrahedral Molecular Geometry (0 Lone Pairs)
When all 4 domains are bonding, the molecular geometry is tetrahedral. All bond angles are exactly 109.5°.
Example: CH₄ (methane)
Carbon forms four single bonds to four hydrogen atoms and carries no lone pairs. All four electron domains are bonding, so both the EDG and the MG are tetrahedral. Every H–C–H angle is 109.5° — not 90°, not 180°, regardless of how flat the Lewis structure looks on paper.
Trigonal Pyramidal Molecular Geometry (1 Lone Pair)
When one of the 4 tetrahedral domains is a lone pair, the three bonded atoms form the base of a pyramid and the central atom sits at the apex — but the fourth tetrahedral position is a lone pair, not an atom. The resulting atom-only shape is called trigonal pyramidal.
Example: NH₃ (ammonia)
Nitrogen has three bonding domains (to three hydrogen atoms) and one lone pair. EDG: tetrahedral. MG: trigonal pyramidal. The lone pair's greater repulsion pushes the H–N–H bond angles slightly below the ideal 109.5° — the actual angle is approximately 107°.
Bent Molecular Geometry (2 Lone Pairs)
When 2 of the 4 tetrahedral domains are lone pairs, only two bonded atoms remain. The three-atom arrangement (atom–central atom–atom) is again called bent — the same geometry name encountered with 3 total domains, but now derived from a tetrahedral EDG. Bond angles are compressed further, to approximately 104.5°.
Example: H₂O (water)
Oxygen has two bonding domains (to two hydrogen atoms) and two lone pairs. EDG: tetrahedral. MG: bent. The two lone pairs exert even greater combined repulsion, compressing the H–O–H angle to approximately 104.5°.
The lone-pair compression trend: Each lone pair added to a tetrahedral central atom depresses the bond angle by roughly 2.5°. CH₄: exactly 109.5° (no lone pairs). NH₃: ~107° (one lone pair). H₂O: ~104.5° (two lone pairs). You are not expected to memorize these exact values — just know the trend: more lone pairs mean smaller bond angles.
| Molecule | Bonding Domains | Lone Pairs | EDG | Molecular Geometry | Bond Angle |
|---|---|---|---|---|---|
| CH₄ | 4 | 0 | Tetrahedral | Tetrahedral | 109.5° |
| NH₃ | 3 | 1 | Tetrahedral | Trigonal Pyramidal | ~107° |
| H₂O | 2 | 2 | Tetrahedral | Bent | ~104.5° |
Molecular Shapes: 5 Electron Domains
Five electron domains require an expanded octet, which is only available to central atoms in the third period or beyond. The electron domain geometry is trigonal bipyramidal, and 4 distinct molecular geometries arise depending on how many lone pairs are present.
Axial vs. Equatorial Positions
Not all 5 positions in a trigonal bipyramidal arrangement are equivalent, and this distinction is critical for determining where lone pairs prefer to sit.
- Equatorial positions (3): The three domains in the horizontal triangular plane. They are 120° apart from each other and 90° from the axial domains.
- Axial positions (2): The two domains pointing straight up and straight down, 180° apart from each other and 90° from all three equatorial domains.
Lone pairs always go equatorial first in a trigonal bipyramidal arrangement. An equatorial lone pair has only 2 neighbors at 90° (the two axial domains). An axial lone pair would have 3 neighbors at 90° (all three equatorial domains). Since 90° interactions are the most destabilizing, minimizing the number of them is energetically favorable — so lone pairs park in equatorial seats.
Trigonal Bipyramidal Molecular Geometry (0 Lone Pairs)
When all 5 domains are bonding, the MG is trigonal bipyramidal, identical to the EDG. Bond angles are 120° (equatorial–equatorial), 90° (equatorial–axial), and 180° (axial–axial).
Seesaw Molecular Geometry (1 Lone Pair)
One lone pair occupies an equatorial position. The four remaining bonded atoms form a shape that resembles a children's seesaw (also called a sawhorse): two atoms roughly 180° apart (the former axials) with two more atoms angled outward below them. The molecular geometry is called seesaw (or sawhorse — both names are in use).
T-Shaped Molecular Geometry (2 Lone Pairs)
Two lone pairs fill the two equatorial positions. The three remaining bonded atoms — one equatorial and two axial — form a perfect capital T. The molecular geometry is T-shaped.
Linear Molecular Geometry (3 Lone Pairs)
All 3 equatorial positions are occupied by lone pairs, leaving only the two axial bonded atoms. These two atoms are 180° apart, giving a linear molecular geometry — the same name as the two-domain case, but arising here from a five-domain trigonal bipyramidal EDG.
Geometry names repeat across different EDGs: "Linear" appears under both 2 and 5 electron domains. "Bent" appears under both 3 and 4 electron domains. The molecular geometry name alone does not tell you the total number of electron domains — you always need both the EDG and the MG to fully describe the molecule.
| Bonding Domains | Lone Pairs | EDG | Molecular Geometry |
|---|---|---|---|
| 5 | 0 | Trigonal Bipyramidal | Trigonal Bipyramidal |
| 4 | 1 (equatorial) | Trigonal Bipyramidal | Seesaw |
| 3 | 2 (equatorial) | Trigonal Bipyramidal | T-Shaped |
| 2 | 3 (all equatorial) | Trigonal Bipyramidal | Linear |
Molecular Shapes: 6 Electron Domains
Six electron domains also require an expanded octet. The electron domain geometry is octahedral, and 3 molecular geometries are possible: octahedral, square pyramidal, and square planar.
All 6 Positions Are Equivalent in Octahedral Geometry
Unlike the trigonal bipyramidal arrangement, all six positions in an octahedral EDG are equivalent. There is no axial/equatorial distinction because rotating the molecule 90° converts what was axial into equatorial and vice versa. This symmetry has an important consequence: when placing lone pairs, any position is equally valid for the first lone pair. For the second lone pair, however, placement matters — and the rule is to put it as far from the first lone pair as possible (180° away).
Octahedral Molecular Geometry (0 Lone Pairs)
When all 6 domains are bonding, the MG is octahedral, matching the EDG. SF₆ is the classic example. All adjacent bond angles are 90°; opposite positions are 180° apart.
Square Pyramidal Molecular Geometry (1 Lone Pair)
One lone pair takes any position (they are all equivalent). The five remaining bonded atoms form a square base with the central atom rising to a single apex — a square-based pyramid. The molecular geometry is square pyramidal. IF₅ (iodine pentafluoride) is a common example.
Square Planar Molecular Geometry (2 Lone Pairs)
Two lone pairs are placed 180° apart from each other (one above, one below) to minimize their mutual repulsion — they are the most repulsive domains and need maximum separation. The four remaining bonded atoms all lie in a single horizontal plane, forming a perfect square. The molecular geometry is square planar. XeF₄ is the textbook example.
Example: Why the lone pairs in XeF₄ go opposite each other
Xenon in XeF₄ has six electron domains: four bonding (to four fluorine atoms) and two lone pairs. The lone pairs carry the greatest repulsion, so they must be as far apart as possible. In an octahedral arrangement, the maximum separation is 180° — directly opposite. Placing both lone pairs on opposite axial positions leaves the four fluorines in the equatorial plane, producing the flat, square planar molecular geometry.
| Bonding Domains | Lone Pairs | EDG | Molecular Geometry |
|---|---|---|---|
| 6 | 0 | Octahedral | Octahedral |
| 5 | 1 | Octahedral | Square Pyramidal |
| 4 | 2 (opposite) | Octahedral | Square Planar |
6-Step Method: Lewis Structure to Molecular Geometry
Now that you have all the individual molecular geometries, here is the 6-step method for converting any Lewis structure into a complete molecular geometry prediction on an exam.
- Draw (or interpret) the Lewis structure. If you are given only a chemical formula — such as XeF₄ — you must first draw the correct Lewis structure before you can determine geometry. Strong Lewis structure skills are the prerequisite for everything in this lesson.
- Identify the central atom and count its electron domains: bonding atoms + lone pairs.
- Determine the EDG from the total domain count using the table above.
- Count lone pairs separately to identify which molecular geometry sub-case applies.
- Name the molecular geometry based on the atom-only arrangement.
- State the bond angles, adjusting for the presence of lone pairs or multiple bonds if needed.
Example: Full 6-step walkthrough for H₂O
Step 1: Lewis structure shows oxygen bonded to two hydrogens, with two lone pairs on oxygen.
Step 2: Central atom is oxygen. Electron domains: 2 bonding + 2 lone pairs = 4 total.
Step 3: Four electron domains → EDG: tetrahedral.
Step 4: Two lone pairs present → MG sub-case for tetrahedral with 2 lone pairs.
Step 5: MG: bent.
Step 6: Ideal tetrahedral angle is 109.5°; two lone pairs compress it to approximately 104.5°.
Exam tip: Questions on molecular geometry frequently start with just a chemical formula, not a Lewis structure. You must draw the Lewis structure yourself as the first step. Practice Lewis structures until you can draw them confidently and quickly — they are the entry point to every molecular geometry problem.
Key Takeaways
- VSEPR theory is built on one idea: electron domains (bonding pairs and lone pairs) repel one another and arrange themselves as far apart as possible around a central atom to produce the lowest-energy molecular geometry.
- An electron domain is any bond to an atom (regardless of whether it is a single, double, or triple bond) or any lone pair. Count total domains to determine the electron domain geometry.
- The 5 electron domain geometries — linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral — are determined by the total number of electron domains (2 through 6).
- The molecular geometry is determined by the positions of atoms only (lone pairs are excluded). If all domains are bonding, the MG name equals the EDG name. Each lone pair shifts the MG to a new name, producing 12 total molecular geometries.
- Bond angles to memorize: 180° (linear), 120° (trigonal planar), 109.5° (tetrahedral), 90/120/180° (trigonal bipyramidal), 90/180° (octahedral). The 109.5° value is among the most frequently tested.
- Lone pairs compress bond angles below the ideal value. Each lone pair added to a tetrahedral center reduces the bond angle by roughly 2.5° (CH₄ 109.5° → NH₃ ~107° → H₂O ~104.5°).
- In trigonal bipyramidal geometry, lone pairs always prefer equatorial positions (fewer 90° interactions). In octahedral geometry, all positions are equivalent, but two lone pairs always sit 180° apart from each other.
- The complete list of 12 molecular geometries you are responsible for: linear, trigonal planar, bent (x2), tetrahedral, trigonal pyramidal, trigonal bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, square planar.
- The workflow is always: Lewis structure → count electron domains → EDG → count lone pairs → MG → bond angles. Mastering Lewis structures first makes every molecular geometry problem straightforward.