2.7 VSEPR and Bond Hybridization

VSEPR theory predicts a molecule's three-dimensional shape by arranging electron domains around a central atom so they repel each other as little as possible. Once you know the shape, you can predict bond angles, hybridization such as $sp$, $sp^2$, and $sp^3$, whether the molecule has a dipole, and how sigma and pi bonds affect bond strength and rotation. For AP Chemistry, start with the Lewis structure before naming geometry or hybridization.

Hybridization and VSEPR in AP Chem

For AP Chemistry, hybridization is a shortcut for connecting electron domains to molecular geometry. Count the electron domains around the atom: 2 domains = sp, 3 domains = sp2, and 4 domains = sp3. Those match ideal bond angles of 180 degrees, 120 degrees, and 109.5 degrees.

VSEPR comes first. Draw the Lewis structure, count domains, and use lone pairs to decide the molecular geometry. Then use the same domain count to identify hybridization for atoms with two, three, or four electron domains. For atoms with more than four electron domains, AP Chemistry expects the shape, but not d-orbital hybridization.

Why This Matters for the AP Chemistry Exam

This topic pulls together everything you built in Unit 2. You take a Lewis structure and use it to explain real molecular properties: shape, bond angles, polarity, relative bond length, and relative bond energy. On the AP Chemistry exam you are expected to construct Lewis diagrams and then support claims about a molecule's structure and behavior using those models, which is exactly the kind of reasoning that shows up in both multiple-choice questions and free-response questions.

Common stumbling points the exam targets directly include picking the wrong number of valence electrons, confusing electron domain geometry with molecular geometry, and mixing up bond angles with shape names. Getting comfortable with VSEPR turns a lot of these into quick, reliable points.

Key Takeaways

• VSEPR uses Coulombic repulsion between electron domains to predict the arrangement around a central atom; lone pairs and bonding pairs both count as domains.
• Electron domain geometry counts all domains, but molecular geometry describes only the positions of atoms, so lone pairs change the shape name.
• Hybridization links directly to geometry: sp is linear (180 degrees), sp2 is trigonal planar (120 degrees), sp3 is tetrahedral (109.5 degrees).
• A single bond is 1 sigma bond, a double bond is 1 sigma + 1 pi, and a triple bond is 1 sigma + 2 pi; more pi bonds mean shorter, stronger bonds and restricted rotation.
• A molecule has a net dipole only when polar bonds do not cancel by symmetry, so geometry matters as much as electronegativity differences.
• Bond length depends on both bond order and atomic radius: higher bond order shortens bonds, and larger atoms lengthen them.

Valence Shell Electron Pair Repulsion (VSEPR)

A correct Lewis structure tells you a molecule's geometry, bond orders, bond lengths, and dipole. VSEPR theory predicts molecular geometry by arranging electron domains so they repel each other as little as possible. It uses Coulombic repulsion between electrons as the basis for predicting where electron pairs sit around a central atom.

Steps for Finding Molecular Geometry

1. Draw the Lewis structure.
2. Count electron domains around the central atom (both bonding groups and lone pairs).
3. Determine the electron domain geometry from the total number of domains.
4. Look at only the atoms (ignore lone pairs) to name the molecular geometry.

A few terms to keep straight:

• Steric number / electron domains: the total number of bonding groups plus lone pairs on the central atom. A double or triple bond still counts as one domain.
• AXE notation: a shorthand where A is the central atom, X is the number of attached atoms, and E is the number of lone pairs. The sum of X and E gives you the electron domain geometry.
• Electron domain geometry: where all domains point, including lone pairs.
• Molecular geometry (shape): where only the atoms sit. This is what you usually report.
• Hybridization: focus on sp, sp2, and sp3. Arrangements with more than four domains exist, but you are only responsible for naming their shapes.

The Main Geometries to Know

Lone pairs take up more space than bonding pairs, so they push bonding pairs closer together and shrink bond angles below the ideal value. That is why the H-N-H angle in NH3 is about 107 degrees instead of 109.5 degrees.

Family 5: Five Electron Domains

When a central atom has 5 electron domains, they arrange in a trigonal bipyramidal electron domain geometry to minimize repulsion. This creates two distinct types of positions:

• Equatorial positions: 3 positions in a plane, 120 degrees apart
• Axial positions: 2 positions perpendicular to that plane, 180 degrees apart from each other

Lone pairs prefer equatorial positions because they have more room there and create the least repulsion with other domains.

Family 6: Six Electron Domains

With 6 electron domains, the electron domain geometry is octahedral, with all positions equivalent at 90 degree angles:

Hybridization involving d orbitals is not assessed on the exam, so for these five- and six-domain cases you only need to name the resulting shape.

Bonding

Sigma and Pi Bonds

Bonds form when atomic orbitals overlap. The type and amount of overlap set the bond's strength and behavior.

Sigma (sigma) bonds form from direct, head-on overlap of orbitals along the internuclear axis (the line connecting two nuclei). This can happen between:

• Two s orbitals
• An s and a p orbital
• Two p orbitals overlapping end-to-end
• Hybrid orbitals (sp, sp2, sp3)

That head-on overlap concentrates electron density between the nuclei, so sigma bonds are the strongest type of covalent bond.

Pi (pi) bonds form from sideways overlap of p orbitals above and below the internuclear axis, creating electron density in two regions instead of right between the nuclei. Because sideways overlap is weaker than head-on overlap, pi bonds have lower bond energy than sigma bonds.

Keep these patterns in mind:

1. A single bond is 1 sigma bond.
2. A double bond is 1 sigma bond and 1 pi bond.
3. A triple bond is 1 sigma bond and 2 pi bonds.

The more pi bonds in a bond:

• The higher the bond energy
• The shorter the bond length
• The more restricted the rotation (single bonds rotate freely, but double and triple bonds cannot rotate because of the pi bonds)

That restriction has a real consequence: it produces geometric isomers (also called cis-trans isomers). These are molecules with the same formula and connectivity but different spatial arrangements because rotation is locked. For example, in 2-butene (CH3CH=CHCH3):

• Cis-2-butene: both methyl groups on the same side of the double bond
• Trans-2-butene: the methyl groups on opposite sides

These are different compounds with different physical properties (boiling points, melting points, dipole moments) even though their molecular formula is identical. The rigidity comes directly from the pi bond.

Worked Example: Counting Sigma and Pi Bonds

Count the sigma bonds and pi bonds in a molecule by treating each single bond as 1 sigma, each double bond as 1 sigma + 1 pi, and each triple bond as 1 sigma + 2 pi.

• A molecule with 1 triple bond and 2 single bonds has 3 sigma bonds and 2 pi bonds (the triple bond gives 1 sigma + 2 pi, and each single bond gives 1 sigma).
• A molecule with 3 double bonds and 9 single bonds has 12 sigma bonds and 3 pi bonds (each double bond gives 1 sigma + 1 pi, and each single bond gives 1 sigma).

Hybridization

Hybridization describes how atomic orbitals combine into new hybrid orbitals that match a molecule's geometry and bonding. You will be tested on three: sp, sp2, and sp3. Hybridization explains how an atom like carbon can form four equivalent sigma bonds in CH4 even though its unhybridized orbitals would not allow that arrangement.

The type of hybridization lines up directly with electron domain geometry and ideal bond angles:

• sp hybridization: linear geometry, bond angles of 180 degrees
  • Example: BeCl2, CO2, HC≡CH (around the triple-bonded carbons)
• sp2 hybridization: trigonal planar geometry, bond angles of 120 degrees
  • Example: BF3, H2C=CH2 (around the double-bonded carbons)
• sp3 hybridization: tetrahedral electron domain geometry, ideal bond angles of 109.5 degrees
  • Example: CH4, NH3 (lone pairs reduce the H-N-H angle to about 107 degrees)

To find hybridization fast, count electron domains on the central atom: 2 domains is sp, 3 domains is sp2, 4 domains is sp3.

Dipole Moments

A molecule has a dipole moment when electron density is distributed unevenly, separating positive and negative charge. To check for one:

1. See whether the molecule contains polar bonds (bonds between atoms of different electronegativity).
2. Consider the molecular geometry, since symmetrical molecules can have polar bonds that still cancel.
3. Draw a dipole arrow from delta-plus to delta-minus for each polar bond.
4. Add the vectors. If they cancel, the molecule is nonpolar; if not, it has a net dipole moment.

For example, CO2 has two polar C=O bonds, but its linear geometry makes the dipoles cancel, so it is nonpolar. H2O has two polar O-H bonds in a bent geometry, so the dipoles do not cancel and the molecule has a net dipole. Geometry matters just as much as the polarity of individual bonds.

Bond Length and Atomic Radius

Bond length depends on bond order and on atomic radius. Larger atoms form longer bonds because their valence electrons sit farther from the nucleus. Comparing similar bonds:

• C-C bonds are shorter than Si-Si bonds
• C-F bonds are shorter than C-Cl bonds
• H-F bonds are shorter than H-I bonds

This follows the periodic trend for atomic radius: moving down a group, atoms get larger, so bonds get longer. When you analyze bond length, weigh both the bond order (more bonds means shorter distance) and the sizes of the atoms involved.

How to Use This on the AP Chemistry Exam

MCQ

• Translate quickly from formula to shape. Draw a fast Lewis structure, count domains, then name the molecular geometry and read off the ideal bond angle.
• Watch the difference between electron domain geometry and molecular geometry. A question that gives four domains with one lone pair wants trigonal pyramidal, not tetrahedral.
• Use domain count to assign hybridization without overthinking it: 2 domains sp, 3 domains sp2, 4 domains sp3.
• For polarity questions, check symmetry. Polar bonds that point in canceling directions give a nonpolar molecule.

Free Response

• Always start with a correct Lewis structure. Many shape, hybridization, and polarity points depend on it, and credit often carries forward if your later answers stay consistent with the structure you drew.
• When asked to explain equal bond lengths, bring in resonance: if a molecule has equivalent resonance structures, the actual bonds are identical with the same bond order.
• Justify, do not just name. If a prompt asks why one C-to-C bond is shortest, connect bond order to bond length (a triple bond is shorter than a double or single bond).
• For hybridization answers, match the count of electron domains on the specific atom named in the question, since different atoms in the same molecule can have different hybridizations.

Common Trap

State the molecular geometry, not the electron domain geometry, unless the question specifically asks about electron pair arrangement. Naming "tetrahedral" for water instead of "bent" is a frequent lost point.

Common Misconceptions

• Electron domain geometry equals molecular geometry. Lone pairs count toward the domain arrangement but are invisible in the shape name. Four domains can be tetrahedral, trigonal pyramidal, or bent depending on how many lone pairs there are.
• A multiple bond counts as more than one domain. A double or triple bond counts as a single electron domain for VSEPR purposes.
• Bond angles always hit the ideal value. Lone pairs repel more strongly than bonding pairs and compress angles below the ideal, which is why water is about 104.5 degrees and ammonia about 107 degrees.
• Polar bonds always make a polar molecule. Symmetry can cancel bond dipoles, as in CO2 and CCl4, leaving a nonpolar molecule.
• Pi bonds are stronger than sigma bonds. Sigma bonds have greater bond energy because head-on overlap is more effective than the sideways overlap of pi bonds.
• Only bond order sets bond length. Atomic radius matters too, so a single C-Cl bond can be longer than a single C-F bond even though both are single bonds.

zation in AP Chemistry?

Hybridization describes how valence orbitals are arranged around an atom to match its bonding geometry. On the AP Chemistry exam, you need to use sp, sp2, and sp3 hybridization, not derive or draw hybrid orbitals.

How do you find hybridization from VSEPR?

Draw a Lewis structure, count electron domains around the atom, then match the count to hybridization. Two domains is sp, three domains is sp2, and four domains is sp3.

What is the VSEPR chart for AP Chem?

The core AP Chem VSEPR pattern is 2 domains: linear, 180 degrees, sp; 3 domains: trigonal planar or bent, about 120 degrees, sp2; 4 domains: tetrahedral, trigonal pyramidal, or bent, about 109.5 degrees or less, sp3.

Does AP Chemistry test sp3d or sp3d2 hybridization?

No. AP Chemistry does not assess hybridization involving d orbitals. For central atoms with more than four electron domains, you are responsible for the resulting molecular shape, not d-orbital hybridization.

How do sigma and pi bonds relate to double and triple bonds?

A single bond has 1 sigma bond, a double bond has 1 sigma and 1 pi bond, and a triple bond has 1 sigma and 2 pi bonds. Pi bonds shorten bonds and restrict rotation.

How do you know if a molecule has a dipole moment?

Check both bond polarity and molecular geometry. A molecule has a net dipole if its polar bond dipoles do not cancel because of the molecule's shape.

Related AP Chemistry Guides

• 2.1 Types of Chemical Bonds
• Unit 2 Overview: Compound Structure and Properties
• 2.2 Intramolecular Force and Potential Energy
• 2.3 Structure of Ionic Solids
• 2.4 Structure of Metals and Alloys
• 2.6 Resonance and Formal Charge