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Organic Chemistry 1 · Structure and Bonding

Molecular Geometry and Hybridization

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

predicts shape by assuming electron domains (bonds and lone pairs) spread as far apart as possible, giving tetrahedral (4 domains), trigonal planar (3), and linear (2) geometries. describes bonds as overlapping orbitals: carbon hybridizes its orbitals into sp³, sp², or sp sets to match its geometry. Single bonds are one ; double bonds add a ; triple bonds add two. More s character shortens and strengthens bonds and increases acidity.

Why this matters

determines whether a drug fits its protein target — the "lock-and-key" of pharmacology. A drug's three-dimensional shape and the rigid planar geometry of its sp² and sp regions control docking into an enzyme's active site. The same planarity dictates protein and DNA structure and lets chemists judge whether a candidate can adopt the shape needed to bind its receptor.

The college version

1. VSEPR Theory

VSEPR theory (Valence Shell Electron Pair Repulsion) holds that electron domains — bonds and lone pairs — repel and arrange to minimize that repulsion. counts all domains; molecular geometry counts only atoms (lone pairs occupy space but are invisible in the shape). Two domains give linear (180°), three trigonal planar (120°), four tetrahedral (109.5°). Lone pairs repel more than bonding pairs, compressing angles (water's H–O–H is 104.5°).

2. Valence-Bond Theory and Hybridization

Valence-bond theory describes a covalent bond as the overlap of atomic orbitals, each holding one electron. Hybridization mixes an atom's s and p orbitals into equivalent hybrids pointing in the directions VSEPR demands: sp³ (one s + three p → four orbitals, tetrahedral), sp² (one s + two p → three orbitals plus one leftover p, trigonal planar), sp (one s + one p → two orbitals plus two leftover p, linear). The leftover p orbitals make π bonds.

3. Sigma and Pi Bonds

A sigma (σ) bond comes from head-on overlap along the bond axis; it is cylindrically symmetrical and allows rotation. A pi (π) bond comes from side-by-side overlap of parallel p orbitals above and below the axis. A single bond is one σ; a double bond is one σ + one π; a triple bond is one σ + two π. π bonds lock atoms in place, preventing rotation around double bonds — the origin of cis/trans isomerism.

4. Bond Lengths, Strengths, and the Hybridization–Acidity Relationship

As s character rises (sp³ → sp² → sp), orbitals hold electrons closer to the nucleus, so bonds get shorter and stronger and bond angles open (109.5° → 120° → 180°). The same effect changes acidity: more s character holds the electron pair tighter, stabilizing the conjugate base's lone pair. Hence the : sp C–H (alkyne, pKₐ ≈ 25) is more acidic than sp² C–H (alkene, ≈ 44), which is more acidic than sp³ C–H (alkane, ≈ 50).

How it works

  1. Count electron domains (bonds and lone pairs) around the atom.
  2. Convert the count to geometry: 2 → linear, 3 → trigonal planar, 4 → tetrahedral.
  3. Choose the matching hybridization: sp, sp², or sp³.
  4. Assign σ bonds to hybrids and π bonds to leftover p orbitals.
  5. Apply s-character trends to bond length/strength and acidity.

Common confusions

Do not confuseWithDifference
Electron-domain geometryMolecular geometryDomains include lone pairs; geometry ignores them
HybridizationPhysical mixing of orbitalsIt is mathematical recombination
σ bondπ bondHead-on vs side-by-side overlap
Double bond strongerDouble bond longerMore bonds → stronger and shorter
Atom hybridizationWhole-molecule shapeAn sp² carbon is planar only locally

Memory aids

Remember "Straight, Triangle, Tetra": sp → straight (linear, 180°), sp² → trigonal (three, 120°), sp³ → tetrahedral (four, 109.5°). The exponent plus one equals the number of domains.

Quick review

Topic Recap

VSEPR turns electron-domain counts into the three core geometries — tetrahedral, trigonal planar, and linear. Valence-bond theory rationalizes them through sp³, sp², and sp hybridization, with σ bonds from hybrid overlap and π bonds from leftover p orbitals. Single, double, and triple bonds are σ/π combinations, and s character governs bond angles, lengths, strengths, and C–H acidity. This bridges atomic structure to the reactivity covered in later topics.

Knowledge Check

  1. How many electron domains surround nitrogen in ammonia (NH₃), and what is the electron-domain geometry?
  2. What is the hybridization of each carbon in ethene?
  3. Which bond is longer and weaker — C–C or C=C — and why?
  4. Why can a C=C double bond not rotate freely?
  5. Rank alkane (sp³), alkene (sp²), and alkyne (sp) C–H bonds by acidity.

Answers and Rationales

  1. Four domains (three N–H bonds + one lone pair) → tetrahedral electron-domain geometry; molecular geometry is trigonal pyramidal (lone pair uncounted).
  2. Each carbon is sp² (three domains: two C–H bonds and one C=C double bond counted as a single domain).
  3. The single bond is longer and weaker: a double bond's extra π overlap pulls atoms closer, shortening and strengthening it.
  4. Rotation would break the side-by-side π overlap; the p orbitals must stay parallel, locking the double bond planar.
  5. Alkyne (sp) > alkene (sp²) > alkane (sp³) — more s character stabilizes the conjugate base's lone pair.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine tying balloons together: they push apart as far as possible — four form a tetrahedron, three a flat triangle, two a straight line. That is VSEPR: electron "balloons" maximize separation. For hybridization, think of mixing letters to make new sounds — carbon combines its s and p orbitals into hybrids matching the shape it needs: sp³ for four balloons, sp² for three, sp for two. The analogy stops being exact because hybridization is a mathematical recombination of wave functions, not a physical mixing of objects, and the hybrids are only approximate descriptions of where electrons are.

Simple Example

Methane, CH₄, has four C–H bonds in a tetrahedron (109.5°); carbon's 2s and three 2p orbitals combine into four equivalent sp³ hybrids. Ethene (CH₂=CH₂) is trigonal planar at each carbon (sp²); ethyne (HC≡CH) is linear (sp).

Worked example

We assign geometry and hybridization to the carbons of acetaldehyde, CH₃CHO (a structure assignment; no electrons move).

  1. Count electron domains per carbon. The methyl carbon has four single bonds → four domains → tetrahedral, sp³.
  2. The carbonyl carbon has three domains — C–C, C–H, and C=O (the double bond counts as one domain despite holding four electrons) → trigonal planar, sp².
  3. Assign bonds: three σ bonds from sp² overlap, plus one π bond from the leftover p orbital overlapping oxygen's p orbital (C=O).
  4. Predict angles: ~109.5° at the methyl carbon, ~120° at the carbonyl carbon.
  5. Read the consequences: the carbonyl carbon is planar, its π bond blocks rotation, and its sp² hybridization makes the aldehyde C–H more acidic than a normal alkane C–H.

Key takeaways

  • High yield: VSEPR counts a double or triple bond as one domain.
  • High yield: sp³ = tetrahedral (109.5°), sp² = trigonal planar (120°), sp = linear (180°).
  • High yield: Double bond = σ + π; triple bond = σ + 2π.
  • High yield: π bonds block rotation — the origin of cis/trans alkene isomerism.
  • High yield: C–H acidity: sp (pKₐ ≈ 25) > sp² (≈ 44) > sp³ (≈ 50).
  • Lone pairs occupy a domain but are absent from molecular geometry.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice Organic Chemistry 1

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Use VSEPR theory to predict electron-domain and molecular geometry (linear, trigonal planar, tetrahedral).
  • Explain how valence-bond theory combines atomic orbitals into sp³, sp², and sp hybrids.
  • Distinguish sigma (σ) and pi (π) bonds and relate them to single, double, and triple bonds.
  • Connect bond angles, lengths, and strengths to hybridization, and explain the hybridization–acidity relationship.

Key vocabulary

VSEPR theory
Electron domains repel to minimize repulsion
Electron-domain geometry
Arrangement of all domains (bonds + lone pairs)
Molecular geometry
Arrangement of only the atoms
Tetrahedral / trigonal planar / linear
4, 3, and 2-domain geometries
Valence-bond theory
Bonds form by orbital overlap
Hybridization (sp³, sp², sp)
Mixing s and p orbitals into hybrids
Sigma (σ) bond
Head-on overlap along the bond axis
Pi (π) bond
Side-by-side p-orbital overlap
Bond angle / length / strength
Geometric/energetic bond descriptors
Single / double / triple bond
1σ; 1σ + 1π; 1σ + 2π
Hybridization–acidity relationship
More s character → more acidic C–H

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