Prepare for AP Chemistry Topic 2.7 with this free practice quiz. Test your understanding of molecular geometry, bond angles, and orbital hybridization to maximize your AP score.
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The Pre-Quiz Review
VSEPR Theory
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular shape by assuming that electron domains around a central atom repel one another and arrange themselves as far apart as possible.
For AP Chemistry, the key is to distinguish between:
- Electron-domain geometry: arrangement of all electron domains around the central atom.
- Molecular geometry: arrangement of bonded atoms only; lone pairs are not named in the molecular geometry.
An electron domain can be:
- A single bond
- A double bond
- A triple bond
- A lone pair
Each counts as one electron domain. Therefore, a C=O double bond counts as one domain, not two.
Lone pairs generally repel more strongly than bonding pairs because their electron density is concentrated closer to the central atom. Repulsion strength follows:
lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair
This is important when predicting bond-angle distortions.
Counting Electron Domains
To determine geometry, first identify the central atom and count its electron domains.
| Electron domains | Electron-domain geometry | Ideal bond angles |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90°, 120°, 180° |
| 6 | Octahedral | 90°, 180° |
Multiple bonds count as one electron domain, but they can exert greater repulsion than a single bond and may cause nearby bond angles to differ from the ideal value.
Two Electron Domains
Two electron domains produce a linear electron-domain geometry.
Typical arrangement: AX₂
Examples: CO₂, BeCl₂
Bond angle: 180°
For AX₂, there are no lone pairs on the central atom, so the molecular geometry is also linear.
Three Electron Domains
Three electron domains produce a trigonal planar electron-domain geometry.
Typical arrangement: AX₃
Example: BF₃
Bond angle: approximately 120°
If one domain is a lone pair, the arrangement becomes AX₂E:
- Electron-domain geometry: trigonal planar
- Molecular geometry: bent
- Bond angle: less than 120°
SO₂ is a common example. Its resonance structures contain a central S atom with three electron domains, giving a bent molecular shape.
Four Electron Domains
Four electron domains produce a tetrahedral electron-domain geometry.
Typical arrangement: AX₄
Example: CH₄
Bond angle: 109.5°
If one electron domain is a lone pair, the arrangement is AX₃E:
- Electron-domain geometry: tetrahedral
- Molecular geometry: trigonal pyramidal
- Bond angle: approximately 107° for NH₃
If two domains are lone pairs, the arrangement is AX₂E₂:
- Electron-domain geometry: tetrahedral
- Molecular geometry: bent
- Bond angle: approximately 104.5° for H₂O
The decrease from 109.5° occurs because lone pairs repel more strongly than bonding pairs.
Five Electron Domains
Five electron domains produce a trigonal bipyramidal electron-domain geometry.
The positions are:
- 2 axial positions
- 3 equatorial positions
Equatorial positions experience fewer 90° interactions, so lone pairs preferentially occupy equatorial positions.
Important molecular geometries include:
AX₅:
- Electron-domain geometry: trigonal bipyramidal
- Molecular geometry: trigonal bipyramidal
- Angles: 90°, 120°, 180°
AX₄E:
- Electron-domain geometry: trigonal bipyramidal
- Molecular geometry: seesaw
AX₃E₂:
- Electron-domain geometry: trigonal bipyramidal
- Molecular geometry: T-shaped
AX₂E₃:
- Electron-domain geometry: trigonal bipyramidal
- Molecular geometry: linear
This preference for equatorial lone pairs is a frequent AP Chemistry reasoning point.
Six Electron Domains
Six electron domains produce an octahedral electron-domain geometry.
AX₆:
- Molecular geometry: octahedral
- Bond angles: 90° and 180°
AX₅E:
- Molecular geometry: square pyramidal
AX₄E₂:
- Molecular geometry: square planar
For AX₄E₂, the two lone pairs occupy positions opposite each other, minimizing lone-pair repulsion.
Examples of octahedral-family structures include SF₆, BrF₅, and XeF₄.
Molecular Geometry vs. Electron-Domain Geometry
Always determine electron-domain geometry before molecular geometry.
For example, NH₃ has four electron domains around N:
- 3 N–H bonds
- 1 lone pair
Therefore:
Electron-domain geometry → tetrahedral
Molecular geometry → trigonal pyramidal
H₂O also has four electron domains:
- 2 O–H bonds
- 2 lone pairs
Therefore:
Electron-domain geometry → tetrahedral
Molecular geometry → bent
The same electron-domain geometry can produce different molecular geometries depending on the number of lone pairs.
Bond Angles
Ideal bond angles come directly from electron-domain geometry:
- Linear → 180°
- Trigonal planar → 120°
- Tetrahedral → 109.5°
- Trigonal bipyramidal → 90° and 120°
- Octahedral → 90°
Actual angles can be smaller or otherwise distorted because lone pairs and multiple bonds produce stronger repulsions.
For four electron domains:
CH₄ ≈ 109.5°
NH₃ ≈ 107°
H₂O ≈ 104.5°
This trend reflects increasing lone-pair repulsion:
CH₄ < one lone pair? No—the key comparison is that replacing bonding pairs with lone pairs compresses the remaining bond angles.
Thus:
CH₄: 0 lone pairs → 109.5°
NH₃: 1 lone pair → ~107°
H₂O: 2 lone pairs → ~104.5°
When comparing molecules, do not automatically choose the molecule with the largest number of bonds as having the largest angle. Count electron domains and consider lone pairs.
VSEPR and Molecular Polarity
Molecular geometry is essential for determining molecular polarity.
A molecule can contain polar bonds but still be nonpolar if the bond dipoles cancel because of molecular symmetry.
Examples:
CO₂:
- Polar C=O bonds
- Linear and symmetrical
- Bond dipoles cancel
- Nonpolar molecule
BF₃:
- Polar B–F bonds
- Trigonal planar and symmetrical
- Dipoles cancel
- Nonpolar molecule
H₂O:
- Polar O–H bonds
- Bent geometry
- Dipoles do not cancel
- Polar molecule
CH₄:
- C–H bonds are often treated as weakly polar
- Tetrahedral and symmetrical
- Overall molecule is nonpolar
For AP questions, determine polarity from both bond polarity and molecular geometry, not from bond polarity alone.
Hybridization
Hybridization describes the mixing of atomic orbitals to produce hybrid orbitals used primarily for sigma bonding and accommodating electron domains.
For the common hybridizations in this topic:
| Hybridization | Number of hybrid orbitals | Electron-domain geometry | Typical angle |
|---|---|---|---|
| sp | 2 | Linear | 180° |
| sp² | 3 | Trigonal planar | 120° |
| sp³ | 4 | Tetrahedral | 109.5° |
A quick way to determine hybridization is to count electron domains around the atom:
2 domains → sp
3 domains → sp²
4 domains → sp³
For the introductory AP Chemistry level, this is usually the fastest approach.
sp Hybridization
An sp-hybridized atom has two hybrid orbitals and a linear arrangement.
Example: CO₂
The central carbon has two electron domains:
O=C=O
Therefore:
- Electron domains: 2
- Geometry: linear
- Hybridization of C: sp
- Bond angle: 180°
An sp-hybridized carbon commonly occurs in a triple bond, such as in C₂H₂.
sp² Hybridization
An sp²-hybridized atom has three hybrid orbitals and a trigonal planar arrangement.
Example: BF₃
The central B has three electron domains:
- 3 B–F bonds
- 0 lone pairs
Therefore:
- Electron domains: 3
- Geometry: trigonal planar
- Hybridization: sp²
- Bond angles: approximately 120°
Carbon atoms involved in C=C double bonds are commonly sp²-hybridized.
sp³ Hybridization
An sp³-hybridized atom has four hybrid orbitals and a tetrahedral electron-domain arrangement.
Examples:
CH₄:
- 4 bonding domains
- sp³
- Tetrahedral
- 109.5°
NH₃:
- 3 bonds + 1 lone pair
- sp³
- Tetrahedral electron-domain geometry
- Trigonal pyramidal molecular geometry
H₂O:
- 2 bonds + 2 lone pairs
- sp³
- Tetrahedral electron-domain geometry
- Bent molecular geometry
Thus, sp³ does not necessarily mean that the molecule itself is tetrahedral. It means the atom has four electron domains.
Sigma and Pi Bonds
Hybrid orbitals are associated primarily with sigma (σ) bonds. Pi (π) bonds form from the side-to-side overlap of unhybridized p orbitals.
Bond composition:
Single bond:
- 1 σ bond
Double bond:
- 1 σ + 1 π
Triple bond:
- 1 σ + 2 π
A carbon-carbon single bond allows rotation more readily because it consists of one σ bond. Double and triple bonds restrict rotation because breaking the π component would be required for free rotation.
The number of electron domains is based on the number of atoms/groups bonded to the central atom, not the total number of σ and π bonds.
For example, in CO₂, each C=O double bond counts as one electron domain. Carbon therefore has two electron domains and is sp-hybridized.
Connecting VSEPR and Hybridization
These concepts are closely connected:
2 electron domains → linear → sp
3 electron domains → trigonal planar → sp²
4 electron domains → tetrahedral → sp³
A reliable AP Chemistry approach is:
- Identify the central atom.
- Draw or interpret the Lewis structure.
- Count electron domains around the central atom.
- Determine electron-domain geometry.
- Account for lone pairs to determine molecular geometry.
- Estimate bond angles.
- Assign hybridization from the number of electron domains.
- If necessary, use symmetry and geometry to determine molecular polarity.
For example, consider NH₃:
Lewis structure → 3 N–H bonds + 1 lone pair
Electron domains → 4
Electron-domain geometry → tetrahedral
Molecular geometry → trigonal pyramidal
Approximate bond angle → 107°
Hybridization of N → sp³
Molecular polarity → polar
For CO₂:
Lewis structure → 2 C=O bonds
Electron domains → 2
Electron-domain geometry → linear
Molecular geometry → linear
Bond angle → 180°
Hybridization of C → sp
Molecular polarity → nonpolar because the molecule is symmetrical
The key AP skill is not memorizing isolated shapes. It is moving consistently from electron count → electron domains → geometry → bond angles → hybridization and, when relevant, polarity.