AP Chemistry Hybridization

Assigning sp, sp², and sp³ to every atom and counting sigma and pi bonds, in four worked examples.

Hybridization questions rarely stop at one central atom. A typical free response hands you a molecule with several interesting atoms and asks you to label the hybridization of each and count the sigma and pi bonds. The method is mechanical once the Lewis structure is right, and the Unit 2 review covers where it fits in the unit. This page works the multi-atom cases step by step, since those are the ones that cost points.

How to Assign Hybridization and Count Sigma and Pi Bonds

1. Draw the Lewis structure with all bonds and lone pairs, as in the Lewis structures guide.
2. Count the electron domains on the atom. Each bonded atom is one domain, whatever the bond order, and each lone pair is one domain.
3. Match the count to the hybridization.
4. Count sigma and pi bonds. Every bond contains exactly one sigma bond; any additional bonds in a double or triple bond are pi.

The course does not assess hybridization involving d orbitals. For an atom with five or six domains you give the shape from the VSEPR chart, not a hybrid label. The same Unit 2 review also notes that you state the label and angle and do not derive the mixing of orbitals.

Worked Example 1: Hybridization of a Central Atom From Its Electron Domains

Give the hybridization of the central atom in CH₄, NH₃, H₂O, BF₃, and CO₂.

CH₄: 4 bonded atoms, 0 lone pairs = 4 domains, so sp³.
NH₃: 3 bonded atoms + 1 lone pair = 4 domains, so sp³ (bond angle about 107°).
H₂O: 2 bonded atoms + 2 lone pairs = 4 domains, so sp³ (bond angle about 104.5°).
BF₃: 3 bonded atoms, 0 lone pairs = 3 domains, so sp².
CO₂: 2 bonded atoms (each a double bond), 0 lone pairs = 2 domains, so sp.

Water and ammonia are both sp³ even though they are not tetrahedral molecules: the lone pairs count as domains, which is why the bond angle in water is smaller than 109.5°.

Worked Example 2: Sigma and Pi Bonds in Ethene, Ethyne, and HCN

For C₂H₄ (H₂C=CH₂), C₂H₂ (HC≡CH), and HCN (H–C≡N), give the hybridization of each carbon and nitrogen and the number of sigma and pi bonds.

Ethene: each carbon has 2 H and 1 C (a double bond, one domain) = 3 domains, so each carbon is sp². Bonds: 4 C–H (4 sigma) + the C=C (1 sigma + 1 pi) = 5 sigma, 1 pi.
Ethyne: each carbon has 1 H and 1 C (a triple bond, one domain) = 2 domains, so each carbon is sp. Bonds: 2 C–H (2 sigma) + the C≡C (1 sigma + 2 pi) = 3 sigma, 2 pi.
HCN: carbon has 1 H and 1 N (triple bond) = 2 domains, sp. Nitrogen has the triple-bonded carbon plus 1 lone pair = 2 domains, sp. Bonds: C–H (1 sigma) + C≡N (1 sigma + 2 pi) = 2 sigma, 2 pi.

The leftover p orbital on each sp² carbon, perpendicular to the plane of the three sigma bonds, overlaps side by side with its neighbor to form the pi bond, as the diagram shows.

Worked Example 3: Labeling Every Carbon and Counting All Bonds in Acetic Acid

Acetic acid is CH₃–C(=O)–O–H. Give the hybridization of each carbon and the total numbers of sigma and pi bonds.

CH₃ carbon: 3 H + 1 C = 4 domains, sp³.
Carbonyl carbon: bonded to CH₃ carbon, to the =O (one domain), and to the –OH oxygen = 3 domains, sp².
Bond count: single bonds are 3 C–H, 1 C–C, 1 C–O, and 1 O–H = 6, each 1 sigma. The C=O adds 1 sigma + 1 pi. Total: 6 + 1 = 7 sigma and 1 pi.

When a molecule has several centers, work through it one atom at a time and keep a running tally of sigma and pi bonds. A mismatch between your total and the number of bonds in the Lewis structure is a quick error check: this molecule has 8 bonds, 7 sigma plus 1 pi.

Worked Example 4: Explaining Why Rotation Is Locked Around a Double Bond

Rotation about the C–C bond in ethane is easy, but rotation about the C=C bond in ethene is not. Explain.

The single bond in ethane is a sigma bond, formed by head-on overlap along the bond axis. Spinning one end about that axis does not change the overlap, so the groups rotate freely. The C=C bond in ethene also contains a pi bond, which comes from side-by-side overlap of p orbitals above and below the plane. Twisting one end of the molecule would turn those p orbitals out of alignment and break the overlap, which costs energy. The pi bond therefore locks the two ends of the molecule in place, which is what makes the two sides of a double bond fixed in position.

Bond strength follows the same logic: a double bond is shorter and stronger than a single bond, as covered in bond order.

Five Hybridization Practice Questions With Answers

1. What is the hybridization of beryllium in BeCl₂?
   Show answer

   sp. Be has 2 bonded atoms and no lone pairs, so 2 domains and a linear shape.
2. For propene, CH₂=CH–CH₃, give the hybridization of each carbon and the number of sigma and pi bonds.
   Show answer

   The two double-bonded carbons are sp² and the CH₃ carbon is sp³. There are 8 sigma bonds (6 C–H, 1 C–C, and the sigma of the double bond) and 1 pi bond.
3. What is the hybridization of nitrogen in NH₄⁺?
   Show answer

   sp³. Four bonded hydrogens and no lone pair give 4 domains.
4. How many sigma and pi bonds are in N₂?
   Show answer

   1 sigma and 2 pi, since N≡N is a triple bond.
5. Why is a pi bond generally weaker than the sigma bond between the same two atoms?
   Show answer

   Its side-by-side overlap of p orbitals is less effective than the direct head-on overlap of a sigma bond, so it takes less energy to break.

Common Hybridization Mistakes

• Counting only bonded atoms. Lone pairs are domains too, so water and ammonia are sp³.
• Counting a double or triple bond as two or three domains. A multiple bond is a single domain.
• Calling every atom in a molecule the same hybridization. Each atom with its own set of domains gets its own label.
• Miscounting pi bonds. A double bond has one pi bond and a triple bond has two; the first bond is always sigma.
• Using d-orbital hybrid labels. They are not assessed; give the shape instead.

Hybridization ties together everything in the structure chain, from Lewis structures through shape and polarity. If you want to see how a topic like this one affects an overall result, the AP Chem Score Calculator shows how section scores combine.

Related Resources

• AP Chem Score Calculator
• Unit 2 Review: Compound Structure and Properties
• AP Chemistry Lewis Structures
• AP Chemistry VSEPR Chart
• AP Chemistry Bond Order
• AP Chemistry Molecular Polarity
• AP Chemistry Study Guide

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