VSEPR and Hybridization
Count domains, distinguish electron and atom geometry, and add bond-dipole vectors.
AP Chemistry Topic 2.7 · CED printed p. 51, 52 / PDF p. 58, 59. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Lewis diagrams including lone pairs
- Bond polarity and vectors
- Resonance and bond order
Count domains before naming shape
Use a valid Lewis model to count electron domains around the central atom. A single, double or triple bond is one domain because its electron density occupies one overall direction. A lone pair is one domain too. VSEPR places domains to reduce electron-pair repulsion. Electron-domain geometry includes all domains; molecular geometry names only atom positions. That is why NH3 has tetrahedral electron-domain geometry but trigonal-pyramidal molecular geometry.
Lone-pair density is concentrated near one center and generally crowds bond domains more than bonding density does, reducing some bond angles. Ideal angles are useful references, not exact measured values for every species. For five domains, equatorial lone-pair positions avoid more 90° close contacts; for six domains and two lone pairs, putting the lone pairs opposite gives square-planar atom positions. The following table gives the assessed families; real angles depend on the species.
Name the directions before placing lone pairs. In a trigonal-bipyramidal arrangement, three equatorial directions lie in one plane, 120° apart, like clock positions 12, 4 and 8. Two axial directions point perpendicular to that plane, one above and one below the central atom, 180° apart. Each axial direction makes a 90° contact with all three equatorial directions; each equatorial direction makes 90° contacts with only the two axial directions.
| Position label | Where it points | Number of 90° contacts |
|---|---|---|
| Axial | Above or below the equatorial plane | 3 |
| Equatorial | One of three planar directions 120° apart | 2 |
A lone pair crowds neighboring domains strongly, so an equatorial location reduces its number of close 90° contacts from three to two. Two lone pairs favor two equatorial directions, leaving two axial bonds and one equatorial bond: the T-shaped atom arrangement. Three equatorial lone pairs leave only the two opposite axial atoms, giving a linear molecule. These are spatial repulsion arguments, not merely shape-name memorization.
| Domains around central atom | Lone-pair domains | Electron-domain geometry | Molecular shape | Representative model |
|---|---|---|---|---|
| 2 | 0 | Linear | Linear, 180° | CO2 |
| 3 | 0 | Trigonal planar | Trigonal planar, 120° | BF3 |
| 3 | 1 | Trigonal planar | Bent, usually <120° | SO2 |
| 4 | 0 | Tetrahedral | Tetrahedral, 109.5° | CH4 |
| 4 | 1 | Tetrahedral | Trigonal pyramidal, usually <109.5° | NH3 |
| 4 | 2 | Tetrahedral | Bent, usually <109.5° | H2O |
| 5 | 0 | Trigonal bipyramidal | Trigonal bipyramidal, 90°/120°/180° | PCl5 |
| 5 | 1 | Trigonal bipyramidal | Seesaw | SF4 |
| 5 | 2 | Trigonal bipyramidal | T-shaped | ClF3 |
| 5 | 3 | Trigonal bipyramidal | Linear | XeF2 |
| 6 | 0 | Octahedral | Octahedral, 90°/180° | SF6 |
| 6 | 1 | Octahedral | Square pyramidal | BrF5 |
| 6 | 2 | Octahedral | Square planar | XeF4 |
Shape turns bond dipoles into a molecular dipole
Mark each bond dipole toward the more electronegative atom, then add the arrows as vectors in three dimensions. A vector has a magnitude and a direction; opposite equal vectors cancel, while equal vectors pointing partly the same way reinforce along that shared direction. In linear CO2, the equal C→O arrows point in exactly opposite directions. In bent H2O, the H→O arrows have sideways components that cancel but components toward oxygen that add, leaving a nonzero net dipole. Identical polar bonds can cancel when arranged symmetrically: linear CO2, planar BF3, trigonal-bipyramidal PCl5 and octahedral SF6 have zero net dipole in these ideal molecular models. Bent H2O and pyramidal NH3 lack that cancellation and have net dipoles. A tetrahedral center with different substituents need not be nonpolar; CH3Cl is a counterexample.
For equivalent atom pairs, increasing bond order gives shorter, stronger bonds. Larger bonded atomic radii tend to increase lengths when other factors are comparable. Use the Lewis bond orders together with geometry to explain structure; a multiple bond changes strength and length but still counts as one VSEPR domain.
Hybridization and sigma/pi are useful descriptions
For central atoms with two, three or four domains, AP uses sp, sp2 and sp3 nomenclature, with ideal domain angles 180°, 120° and 109.5°. This labels a bonding model; it is not direct evidence that orbitals physically reorganize in a simple mechanical sequence. For more than four domains, know the molecular shape rather than assigning d-orbital hybridization. Deriving or depicting hybrid orbitals and molecular-orbital diagrams is outside the required assessment.
A sigma (σ) bond comes from overlap along the internuclear direction. A pi (π) bond involves side-to-side overlap in addition to the sigma bond. A single bond has one σ; a double has one σ and one π; a triple has one σ and two π. For a comparable atom pair, head-on sigma overlap is ordinarily greater than sideways pi overlap, giving stronger stabilization and a greater individual bond-energy contribution. Adding a weaker pi contribution to a sigma bond nevertheless strengthens the total multiple bond; do not confuse an individual contribution with the total dissociation energy. Rotating about a double bond would disrupt pi overlap, so free rotation is restricted. When each double-bond atom has two different substituents, different arrangements can give geometric isomers; 2-butene is a familiar example.
Let's work through it
Work it through
Compare CO2, NH3, SF4 and ethene C2H4.
- CO2: central C has two double bonds and no lone pairs → two domains → linear, 180°, sp. Its equal opposite C=O dipoles cancel.
- NH3: N has three bonds and one lone pair → four domains → tetrahedral electron arrangement, trigonal-pyramidal atom geometry; bond angles below the ideal 109.5° and sp3 nomenclature. The N–H dipoles do not cancel.
- SF4: central S has four bonds and one lone pair → five domains. Put the lone pair equatorial in a trigonal-bipyramidal arrangement → seesaw. The geometry is asymmetric, giving a net dipole; no d-hybrid assignment is required.
- Ethene: each C has three domains → planar sp2 description near 120°. Its C=C has one σ and one π; four C–H bonds add four σ, for five σ and one π total. Rotation about C=C is restricted.
Make the picture move
Interactive concept model
Bond polarity needs a three-dimensional shape.
Predict first. Count electron domains first. Then compare electron-domain geometry with molecular geometry and test whether bond dipoles cancel.
Molecular dipole = vector sum of bond dipoles (qualitative model)
On a small screen, scroll inside the graph or diagram to read its labels. The values and explanation also appear below.
CO₂ is linear and nonpolar. The two equal C–O bond dipoles point in opposite directions and cancel.
Read the data and comparisons
| Quantity | Selected molecule |
|---|---|
| Formula | CO₂ |
| Central electron domains | 2 |
| Central lone pairs | 0 |
| Molecular geometry | linear |
| Molecular polarity | Nonpolar: symmetric cancellation |
Macroscopic / data. A symmetric molecule can be nonpolar even when its individual bonds are polar.
Particle view. 2 electron domains surround C; 0 are lone pairs.
Symbolic view. CO₂: μ = 0 in this symmetry model.
Why it works. VSEPR models electron-domain repulsion: arrangements place electron pairs as far apart as possible. A single, double, or triple bond counts as one electron domain. Lone pairs occupy domains but are omitted from the name of molecular geometry. Read dipole cancellation using the stated three-dimensional shape, not the apparent angles in a flat projection.
Conditions and limits. Idealized isolated-molecule VSEPR models; the drawing is a labeled projection, not a measured structure. Dashed bonds show projected directions; distances and arrow lengths have no quantitative meaning. C–H bond polarity is small but points toward carbon. PCl₅ here is gas phase; its solid has different species.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Does a triple bond count as three VSEPR domains?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Does a triple bond count as three VSEPR domains?
Worked answer and reason
No. It occupies one directional domain even though it has one σ and two π bonds.
Domain count concerns directions of density, not the number of electron pairs shared in that direction.
Retrieval 2
Why is H2O polar but CO2 nonpolar despite polar bonds in both?
Worked answer and reason
Water is bent, so its bond dipoles add to a nonzero vector; CO2 is linear with equal opposite vectors.
Molecular polarity depends on geometry and bond dipole directions together.
Retrieval 3
What shape follows six domains with two opposite lone pairs?
Worked answer and reason
Square planar molecular shape within an octahedral electron-domain arrangement.
Removing opposite vertices leaves four atoms in one plane.
Retrieval 4
Can every alkene show two geometric isomers?
Worked answer and reason
No. Each carbon of the double bond must have two different substituents for the two arrangements to be distinct.
Restricted rotation is necessary but substituent identity also matters.
Catch the tempting mistake
- Tetrahedral electron geometry always means tetrahedral molecular shape.
- Lone pairs are omitted from the molecular-shape name, giving pyramidal or bent cases.
- Any molecule with polar bonds is polar.
- Symmetry can cancel bond dipoles; add the vectors in the molecular geometry.
- SF6 requires sp3d2 as an AP explanation.
- For more than four domains, the assessed requirement is shape; d-hybridization is excluded.
- The second line in a double bond is another sigma bond.
- A double bond consists of one sigma and one pi bond.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 2 problem 14 · ap application · multiple
- Unit 2 problem 15 · ap synthesis · particle
- Unit 2 problem 16 · ap application · symbolic
- Unit 2 problem 17 · ap synthesis · multiple
- Unit 2 problem 18 · ap synthesis · multiple
Scientific calculator workspace
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Scientific calculator
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For the teacher
- Require domain count, electron geometry, atom geometry and polarity as separate steps.
- For five- and six-domain cases, ask students to explain lone-pair placement with close-contact repulsion.
- Use bond-level counting and isomer conditions to check that shape naming has not replaced orbital-overlap reasoning.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 2.7.A
- Based on the relationship between Lewis diagrams, VSEPR theory, bond orders, and bond polarities: i. Explain structural properties of molecules. ii. Explain electron properties of molecules.
- 2.7.A.1
- VSEPR theory uses the Coulombic repulsion between electrons as a basis for predicting the arrangement of electron pairs around a central atom.
- 2.7.A.2
- Both Lewis diagrams and VSEPR theory must be used for predicting electronic and structural properties of many covalently bonded molecules and polyatomic ions, including the following: i. Molecular geometry (linear, trigonal planar, tetrahedral, trigonal pyramidal, bent, trigonal bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, square planar) ii. Bond angles iii. Relative bond energies based on bond order iv. Relative bond lengths (multiple bonds, effects of atomic radius) v. Presence of a dipole moment vi. Hybridization of valence orbitals for atoms within a molecule or polyatomic ion
- 2.7.A.3
- The terms “hybridization” and “hybrid atomic orbital” are used to describe the arrangement of electrons around a central atom. When the central atom is sp hybridized, its ideal bond angles are 180°; for sp2 hybridized atoms the bond angles are 120°; and for sp3 hybridized atoms the bond angles are 109.5°.
- 2.7.A.4
- Bond formation is associated with overlap between atomic orbitals. In multiple bonds, such overlap leads to the formation of both sigma and pi bonds. The overlap is stronger in sigma than pi bonds, which is reflected in sigma bonds having greater bond energy than pi bonds. The presence of a pi bond also prevents the rotation of the bond and leads to geometric isomers.
- An understanding of the derivation and depiction of hybrid orbitals will not be assessed on the AP Exam. The course includes the distinction between sigma and pi bonding, the use of VSEPR to explain the shapes of molecules, and the sp, sp^2, and sp^3 nomenclature.
- Hybridization involving d orbitals will not be assessed on the AP Exam. When an atom has more than four pairs of electrons surrounding the central atom, students are only responsible for the shape of the resulting molecule.
- Molecular orbital theory is recommended as a way to provide deeper insight into bonding. However, the AP Exam will neither explicitly assess molecular orbital diagrams, filling of molecular orbitals, nor the distinction between bonding, nonbonding, and antibonding orbitals.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §7.6, Molecular Structure and Polarity — Geometry, electron domains and vector cancellation of molecular dipoles.
- OpenStax Chemistry 2e §8.1, Valence Bond Theory — Corresponding head-on versus sideways orbital overlap and the distinction between individual sigma/pi contributions and total multiple-bond energy. New questions and feedback are independently authored.
Glossary and deeper reading · 9 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Lewis structure
A Lewis structure represents valence electrons using bonds and lone pairs. Its construction requires an electron count, an atom arrangement, and a final check against the total available electrons and charge. For example, water's Lewis structure has two O–H bonds and two lone pairs on oxygen.
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Lone pair
A lone pair is a pair of valence electrons assigned to one atom rather than to a bond in a Lewis model. Lone pairs influence geometry and can participate in interactions or new bonds. For example, oxygen's two lone pairs help explain water's bent molecular shape.
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Vector
A vector is a quantity with a magnitude and a direction, represented by an arrow or by components along chosen axes. Vectors add by adding corresponding components rather than simply adding their magnitudes. For example, an eastward displacement followed by a northward displacement produces a resultant pointing northeast. The zero vector has zero magnitude and no unique direction.
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Resonance in bonding
Resonance uses multiple Lewis contributors with the same atom positions to describe delocalized electrons. The actual species has one electronic structure rather than switching back and forth between the drawings. For example, nitrate's equivalent N–O bonds are described with several resonance contributors.
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Bond order
Bond order describes the degree of bonding between two atoms in a specified bonding model. Localized single, double, and triple bonds have orders one, two, and three, while delocalized models can give fractional values. For example, equivalent bonds in a resonance description can have bonding intermediate between a single and double bond.
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Electron domain
An electron domain is a region of electron density around an atom in the VSEPR model. A bond of any order counts as one domain, and a lone pair counts as another. For example, two bonds and two lone pairs give water's oxygen four domains and a steric number of four.
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Bond dipole
A bond dipole describes a bond's separation of positive and negative charge. Its direction and magnitude depend on the distribution of electron density. For example, an O–H bond has a dipole because its electron density is drawn toward oxygen.
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Atomic orbital
An atomic orbital is a quantum state used to describe an electron in an atom. Its spatial description gives a probability distribution rather than a definite path around the nucleus. For example, an s orbital has a spherical spatial probability distribution, and one orbital can accommodate at most two electrons with opposite spins.
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Sigma bond
A sigma bond has bonding electron density along the line joining two nuclei. It can arise from end-to-end overlap in a localized orbital model. For example, the carbon–carbon bond in ethane is a sigma bond.