Example: CO₂

$O=C=O$

• 2 double bonds = 2 electron domains
• No lone pairs
• Total: 2 electron domains

Steric Number

Steric Number (SN): Total number of electron domains around central atom

$\text{SN} = (\text{bonding domains}) + (\text{lone pairs})$

Alternative:

$\text{SN} = (\text{number of atoms bonded}) + (\text{lone pairs on central atom})$

Examples:

CH₄: 4 bonds + 0 lone pairs = SN 4

NH₃: 3 bonds + 1 lone pair = SN 4

H₂O: 2 bonds + 2 lone pairs = SN 4

All have SN = 4, but different molecular shapes!

Electron Geometry vs. Molecular Geometry

Electron Geometry

Definition: Arrangement of ALL electron domains (bonds + lone pairs)

Determined by: Steric number only

Molecular Geometry

Definition: Arrangement of ATOMS only (ignores lone pairs)

Determined by: Steric number AND number of lone pairs

Key distinction: Lone pairs affect geometry but aren't "seen" in molecular shape

Five Basic Electron Geometries

Based on steric number:

Molecular Geometries by Steric Number

SN = 2 (2 electron domains)

Electron geometry: Linear

Linear: All atoms in straight line

$O=C=O$ (180°)

SN = 3 (3 electron domains)

Electron geometry: Trigonal planar

Trigonal planar: Flat, three atoms around center

$F - B - F$ with third F above (all in plane) $|$ $F$

Bent (from trigonal planar): V-shaped

$O=S=O$ (with lone pair on S, angle ~119°)

SN = 4 (4 electron domains)

Electron geometry: Tetrahedral

Tetrahedral: 3D pyramid shape

CH₄: Carbon at center, 4 H atoms at corners of tetrahedron

Trigonal pyramidal: Pyramid with triangular base

NH₃: N at top, 3 H atoms form triangular base (lone pair on top)

Bent (from tetrahedral): V-shaped

H₂O: O at vertex, 2 H atoms, 2 lone pairs (angle 104.5°)

SN = 5 (5 electron domains)

Electron geometry: Trigonal bipyramidal

Two types of positions:

• Axial: Top and bottom (3 at 90°)
• Equatorial: Middle three (at 120°)

Lone pairs prefer equatorial positions (more space, less repulsion)

Trigonal bipyramidal: Two pyramids joined at base

Seesaw: Like teeter-totter

T-shaped: Like letter T

Linear (from SN 5): Straight line with 3 lone pairs

SN = 6 (6 electron domains)

Electron geometry: Octahedral

All positions are equivalent (all 90° apart)

Octahedral: 8-sided figure, 6 atoms at corners of octahedron

Square pyramidal: Square base with atom above

Square planar: Flat square (lone pairs above and below)

Bond Angle Deviations

General Rules

Repulsion strength: Lone pair-lone pair > Lone pair-bond > Bond-bond

Effect: Lone pairs take up more space than bonding pairs

Result: Bond angles decrease when lone pairs present

Examples of Angle Changes

CH₄ (no lone pairs): 109.5° (ideal tetrahedral)

NH₃ (1 lone pair): ~107° (slightly compressed)

H₂O (2 lone pairs): ~104.5° (more compressed)

Trend: More lone pairs → smaller bond angles

Why Lone Pairs Compress Angles

1. Lone pairs are closer to nucleus (not shared)
2. Lone pairs have more electron density
3. Greater repulsion pushes bonding pairs closer together
4. Bond angles become smaller

Predicting Molecular Geometry - Step by Step

Step 1: Draw Lewis structure

Step 2: Count electron domains on central atom

• Bonds (single, double, triple each = 1 domain)
• Lone pairs (each = 1 domain)

Step 3: Determine steric number (SN)

Step 4: Identify electron geometry from SN

Step 5: Count lone pairs

Step 6: Determine molecular geometry

• Use table based on SN and lone pairs

Step 7: Predict bond angles

• Start with ideal angle for electron geometry
• Decrease if lone pairs present

Example Walkthrough: NH₃

Step 1: Lewis structure

$H - N - H$ with lone pair on N $|$ $H$

Step 2: Count electron domains

• 3 N-H bonds = 3 domains
• 1 lone pair = 1 domain
• Total: 4 domains

Step 3: Steric number SN = 4

Step 4: Electron geometry SN 4 → Tetrahedral electron geometry

Step 5: Count lone pairs 1 lone pair

Step 6: Molecular geometry SN 4, 1 lone pair → Trigonal pyramidal

Step 7: Bond angles

• Ideal tetrahedral: 109.5°
• With 1 lone pair: ~107° (slightly less)

Polarity and Molecular Geometry

Molecular polarity depends on:

1. Bond polarity (electronegativity difference)
2. Molecular geometry (symmetry)

Polar vs. Nonpolar Molecules

Nonpolar molecules:

• Symmetrical geometry
• Bond dipoles cancel out
• Examples: CO₂ (linear), CCl₄ (tetrahedral), BF₃ (trigonal planar)

Polar molecules:

• Asymmetrical geometry
• Bond dipoles do NOT cancel
• Examples: H₂O (bent), NH₃ (trigonal pyramidal), HCl (linear but different atoms)

Key Patterns

Always polar if:

• Lone pairs on central atom with polar bonds
• Asymmetric arrangement

Always nonpolar if:

• All bonds are identical AND symmetrical
• No lone pairs AND all surrounding atoms identical

Examples:

CO₂: O=C=O (linear, symmetrical) → Nonpolar

H₂O: H-O-H (bent due to lone pairs) → Polar

CCl₄: Tetrahedral, all Cl identical → Nonpolar

CHCl₃: Tetrahedral, but H ≠ Cl → Polar

NH₃: Trigonal pyramidal with lone pair → Polar

BF₃: Trigonal planar, all F identical → Nonpolar

Multiple Central Atoms

For molecules with multiple "central" atoms, analyze each separately:

Example: Ethanol (C₂H₅OH)

• Left C: SN 4, tetrahedral
• Right C: SN 4, tetrahedral
• O: SN 4, bent (2 lone pairs)

Common Mistakes to Avoid

1. Counting multiple bonds as multiple domains: Double/triple bonds = 1 domain
2. Confusing electron geometry with molecular geometry: Lone pairs count for electron geometry only
3. Forgetting lone pairs: Always include in steric number
4. Wrong polarity determination: Check BOTH bond polarity AND geometry
5. Ignoring lone pair repulsion: Lone pairs decrease bond angles

Summary Table

LP = Lone Pairs

Applications

1. Predicting molecular properties: Shape affects polarity, boiling point, reactivity
2. Drug design: Molecular shape determines how drugs fit receptors
3. Materials science: Geometry affects crystal structure and material properties
4. Understanding reactivity: Shape determines which atoms can interact

Each O has 2 double bonds and 2 lone pairs C has 2 double bonds (no lone pairs)

Step 2: Identify central atom

Central atom: C (carbon)

Step 3: Count electron domains on C

• 1 double bond to left O = 1 electron domain
• 1 double bond to right O = 1 electron domain
• 0 lone pairs on C

Total electron domains: 2

Remember: Multiple bonds count as ONE domain!

Step 4: Determine steric number

$\text{SN} = 2$

Step 5: Identify electron geometry

SN = 2 → Linear electron geometry

Step 6: Count lone pairs on central atom

C has 0 lone pairs

Step 7: Determine molecular geometry

SN = 2, 0 lone pairs → Linear molecular geometry

Step 8: Predict bond angle

Linear geometry → 180°

Answer:

• Molecular geometry: Linear
• Bond angle: 180°
• Structure: O=C=O (all atoms in straight line)

Verification:

• 2 electron domains ✓
• 0 lone pairs ✓
• Electron geometry = Molecular geometry (no lone pairs) ✓
• Maximum separation of 2 domains = 180° ✓

Note: CO₂ is also nonpolar because the two C=O dipoles point in opposite directions and cancel out due to the linear, symmetrical geometry.