Molecular Structure

Molecular Structure (VSEPR)

Molecular shape follows VSEPR: electron pairs repel and stay as far apart as they can. Bonding and lone pairs give linear, trigonal planar, tetrahedral, trigonal pyramidal, or bent geometries. Lone pairs shrink bond angles, and molecular symmetry decides polar versus nonpolar. Here you pick a structure to compare pair arrangements and polarity.

Why Do Molecules Have Specific Shapes?

💡 Analogy: Tying Balloons

①Tie 2 balloons → linear (180°)

②3 balloons → trigonal planar (120°)

③4 balloons → tetrahedral (109.5°)

④Electron pairs repel like balloons, getting as far apart as possible

⑤This is the heart of VSEPR theory!

VSEPR Molecular Structure Visualization

Select shape0

🔍 Compare the Six Shapes!

①Linear (CO₂): 2 bp, 0 lp → nonpolar

②Trigonal planar (BF₃): 3 bp, 0 lp → nonpolar

③Bent (SO₂): 2 bp, 1 lp → polar

④Tetrahedral (CH₄): 4 bp, 0 lp → nonpolar

⑤Trigonal pyramidal (NH₃): 3 bp, 1 lp → polar

Lone Pairs and Bond Angle

VSEPR Core

Repulsion: lone-lone > lone-bond > bond-bond

Lone pairs occupy more space → bond angle decreases

Bond Angle Comparison

CH₄(109.5°) > NH₃(107°) > H₂O(104.5°)

0 → 1 → 2 lone pairs: angle gradually decreases

💡 Why Lone Pairs Reduce Bond Angles

①Lone pair belongs to one nucleus only → spreads more

②Bonding pair shared between two nuclei → narrower

③Lone pair pushes bonds harder

④Result: angle ↓ (109.5° → 107° → 104.5°)

Polarity Determination

ChartRelation Between Shape and Polarity

Shape	Examples	Symmetric?	Polarity
Linear	CO₂	Yes	Nonpolar
Trigonal planar	BF₃	Yes	Nonpolar
Bent	H₂O, SO₂	No	Polar
Tetrahedral	CH₄, CCl₄	Yes	Nonpolar
Trigonal pyramidal	NH₃	No	Polar

💡 Three Steps to Judge Polarity

①Are individual bonds polar? (electronegativity difference)

②Is the molecule symmetric? (use VSEPR)

③Symmetric → dipole moments cancel → nonpolar

④Asymmetric → dipole sum ≠ 0 → polar

⑤Same bonds can give different polarities depending on shape!

Work It Out

Example 1

Find the shape and bond angle of methane (CH₄).

Count the electron pairs around the central C atom.

4 electron pairs around C (all bonding)

With 0 lone pairs, the 4 bonding pairs spread out maximally.

tetrahedral, 109.5°

▸ tetrahedral, 109.5°

With no lone pairs, the angle is not reduced and stays at 109.5°.

Example 2

Compare the shapes of water (H₂O) and carbon dioxide (CO₂).

H₂O has 2 bonding pairs and 2 lone pairs on the central O.

H₂O: lone-pair repulsion → bent, 104.5°

CO₂ has no lone pairs on central C, only 2 double bonds.

CO₂: no lone pairs → linear, 180°

▸ H₂O bent, CO₂ linear

Even with 2 bonding pairs each, the presence of lone pairs splits the shapes into bent vs linear.

Summary

Determining Molecular Structure

① Count electron pairs on central atom → ② Apply VSEPR

Electron pairs = bonding pairs + lone pairs

2022 KICE mock Chemistry I type, adapted

Which of the following molecules has a linear shape?

①H₂O

②NH₃

③CH₄

④CO₂

⑤SO₂

▸ ④ CO₂

Compare the electron-pair arrangement on each central atom.

Only CO₂ has no lone pairs on its central atom, so it is linear.

CO₂: O=C=O, 0 lone pairs on central C → linear, 180°

🎯 Exam Points

①VSEPR = electron pairs maximize separation

②Lone pairs take more space → bond angles decrease

③CH₄: tetrahedral (nonpolar); NH₃: trigonal pyramidal (polar); H₂O: bent (polar)

④CO₂: linear (nonpolar) vs H₂O: bent (polar)

⑤Molecular polarity = bond polarity + shape symmetry combined

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Covalent Bond

Electronegativity & Polarity

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