3.5.2 Shapes of Molecules Containing Three Electron Pairs

This topic explains the shapes of molecules where the central atom is surrounded by three electron pairs. These can be three bonding pairs (AX₃) or two bonding pairs and one lone pair (AX₂E).

Definition

Three Electron Pairs (AX₃ or AX₂E): A molecule in which the central atom is surrounded by three electron pairs — either three bonding pairs or two bonding pairs and one lone pair.

Main Idea

When a central atom is surrounded by three electron pairs, the electron pairs arrange themselves as far apart as possible to minimize repulsion. The ideal arrangement is trigonal planar with bond angles of 120°. However, if one of the electron pairs is a lone pair, the shape becomes bent (V-shaped) and the bond angle is less than 120° because lone pairs repel more strongly than bonding pairs.

Core Concept

• Three bonding pairs (AX₃): Trigonal planar, 120° bond angle.
• Two bonding pairs + one lone pair (AX₂E): Bent (V-shaped), bond angle < 120°.

The lone pair occupies more space near the central atom than a bonding pair, so it pushes the bonding pairs closer together, reducing the bond angle.

Detailed Explanation

Three Bonding Pairs (AX₃)

Shape: Trigonal Planar

• The central atom is surrounded by three bonding pairs.
• There are no lone pairs on the central atom.
• To minimize repulsion, the three electron pairs are placed 120° apart in a plane.
• The shape of the molecule is trigonal planar.
• The bond angle is 120°.

Example: BCl₃ (Boron Trichloride)

• The central atom, boron, contributes three valence electrons.
• Each chlorine atom contributes seven valence electrons.
• Boron forms three sigma bonds with three chlorine atoms.
• The Lewis electron structure: B with three Cl atoms around it.
• There are three electron pairs around the central boron atom.
• To minimize repulsions, the groups are placed 120° apart.
• The geometry is trigonal planar.
• The bond angle is 120°.

Example: CO₃²⁻ (Carbonate Ion)

• In the carbonate ion, the central atom, carbon, has four valence electrons.
• Each oxygen atom has six valence electrons.
• There are three bonding pairs around the carbon atom.
• The molecular geometry of CO₃²⁻ is trigonal planar.
• The bond angle is 120°.

Two Bonding Pairs and One Lone Pair (AX₂E)

Shape: Bent (V-shaped)

• The central atom is surrounded by two bonding pairs and one lone pair.
• There are three electron pairs in total.
• The electron pair geometry is trigonal planar.
• However, the molecular shape is determined by the positions of the atoms only.
• Since one position is occupied by a lone pair, the shape is bent (V-shaped).
• The bond angle is less than 120°.

Example: SO₂ (Sulfur Dioxide)

• The central atom, sulfur, has 6 valence electrons.
• Each oxygen atom has 6 valence electrons.
• There are two bonding pairs and one lone pair around sulfur.
• The structure is designated as AX₂E.
• Because a lone pair occupies more space near the central atom than a bonding pair, the bonding pairs and lone pairs repel each other.
• In SO₂, we have:
• One BP-BP interaction
• Two LP-BP interactions
• The lone pair repels the bonding pairs more strongly, pushing them closer together.
• The shape is bent (V-shaped).
• The bond angle is less than 120° (approximately 119°).

Comparison of AX₃ and AX₂E

Repulsion Order

LP-LP > LP-BP > BP-BP

• Lone pair-lone pair repulsion is the strongest.
• Lone pair-bond pair repulsion is intermediate.
• Bond pair-bond pair repulsion is the weakest.

This is why the bond angle in SO₂ (AX₂E) is less than 120°.

Intermediate Level

AX₃ Molecules (Three Bonding Pairs)

• Central atom: A
• Three bonded atoms: X
• No lone pairs on A
• Shape: Trigonal planar
• Bond angle: 120°

• Boron has 3 valence electrons.
• Each chlorine contributes 1 electron for bonding.
• Total electron pairs around B: 3
• All are bonding pairs.
• Arrangement: 120° apart in a plane.
• Shape: Trigonal planar.

• Carbon has 4 valence electrons.
• Each oxygen contributes electrons for bonding.
• Three bonding pairs around carbon.
• Shape: Trigonal planar.

AX₂E Molecules (Two Bonding Pairs + One Lone Pair)

• Central atom: A
• Two bonded atoms: X
• One lone pair: E
• Shape: Bent (V-shaped)
• Bond angle: < 120°

• Sulfur has 6 valence electrons.
• Each oxygen contributes electrons for bonding.
• Two bonding pairs and one lone pair around sulfur.
• Lone pair repels more strongly.
• Shape: Bent (V-shaped).
• Bond angle: < 120° (approximately 119°).

Table of Three Electron Pair Molecules

Important Points

• 1. AX₃ molecules have three bonding pairs and no lone pairs.
• 2. Shape: Trigonal planar.
• 3. Bond angle: 120°.
• 4. Example: BCl₃, CO₃²⁻.
• 5. AX₂E molecules have two bonding pairs and one lone pair.
• 6. Shape: Bent (V-shaped).
• 7. Bond angle: < 120°.
• 8. Example: SO₂, O₃.
• 9. Lone pairs repel more strongly than bonding pairs.

Remember

• - AX₃: Three bonding pairs, no lone pairs
• - Shape: Trigonal planar
• - Bond angle: 120°
• - Example: BCl₃, CO₃²⁻
• - AX₂E: Two bonding pairs, one lone pair
• - Shape: Bent (V-shaped)
• - Bond angle: < 120°
• - Example: SO₂, O₃
• - Repulsion order: LP-LP > LP-BP > BP-BP

Common Mistakes

• 1. Thinking AX₂E is trigonal planar: The molecular shape is bent because the lone pair is not counted in the molecular shape.
• 2. Forgetting that lone pairs reduce bond angles: Lone pairs repel more strongly than bonding pairs.
• 3. Confusing electron pair geometry with molecular geometry: Electron pair geometry includes lone pairs; molecular geometry considers only atoms.
• 4. Ignoring the repulsion order: LP-LP > LP-BP > BP-BP.

Quick Revision

• AX₃ molecules have three bonding pairs and no lone pairs.
• Shape: Trigonal planar.
• Bond angle: 120°.
• Example: BCl₃, CO₃²⁻.
• AX₂E molecules have two bonding pairs and one lone pair.
• Shape: Bent (V-shaped).
• Bond angle: < 120°.
• Example: SO₂, O₃.
• Lone pairs repel more strongly than bonding pairs.

60-Second Revision

• AX₃: Three bonding pairs, no lone pairs
• Shape: Trigonal planar
• Bond angle: 120°
• Example: BCl₃, CO₃²⁻
• AX₂E: Two bonding pairs, one lone pair
• Shape: Bent (V-shaped)
• Bond angle: < 120°
• Example: SO₂, O₃
• Repulsion order: LP-LP > LP-BP > BP-BP

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