3.5.3 Shapes of Molecules Containing Four Electron Pairs

This topic explains the shapes of molecules where the central atom is surrounded by four electron pairs. These can be four bonding pairs (AX₄), three bonding pairs and one lone pair (AX₃E), or two bonding pairs and two lone pairs (AX₂E₂).

Definition

Four Electron Pairs (AX₄, AX₃E, AX₂E₂): A molecule in which the central atom is surrounded by four electron pairs — either four bonding pairs, three bonding pairs and one lone pair, or two bonding pairs and two lone pairs.

Main Idea

When a central atom is surrounded by four electron pairs, the electron pairs arrange themselves as far apart as possible to minimize repulsion. The ideal arrangement is tetrahedral with bond angles of 109.5°. However, if one or more of the electron pairs are lone pairs, the molecular shape changes and the bond angles decrease because lone pairs repel more strongly than bonding pairs.

Core Concept

• Four bonding pairs (AX₄): Tetrahedral, 109.5° bond angle.
• Three bonding pairs + one lone pair (AX₃E): Trigonal pyramidal, bond angle < 109.5°.
• Two bonding pairs + two lone pairs (AX₂E₂): Bent (V-shaped), bond angle < 109.5°.

The more lone pairs, the greater the reduction in bond angle.

Detailed Explanation

Four Bonding Pairs (AX₄)

Shape: Tetrahedral

• The central atom is surrounded by four bonding pairs.
• There are no lone pairs on the central atom.
• To minimize repulsion, the four electron pairs are arranged in a tetrahedral geometry.
• The shape of the molecule is tetrahedral.
• The bond angle is 109.5°.

Example: CH₄ (Methane)

• Carbon has 4 valence electrons.
• Each hydrogen contributes 1 valence electron.
• Carbon forms four sigma bonds with four hydrogen atoms.
• The Lewis structure: C with four H atoms around it.
• There are four bonding pairs around the central carbon atom.
• The arrangement that minimizes repulsion is tetrahedral.
• The bond angle is 109.5°.
• The shape is tetrahedral.

Three Bonding Pairs and One Lone Pair (AX₃E)

Shape: Trigonal Pyramidal

• The central atom is surrounded by three bonding pairs and one lone pair.
• There are four electron pairs in total.
• The electron pair geometry is tetrahedral.
• 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 trigonal pyramidal.
• The bond angle is less than 109.5°.

Example: NH₃ (Ammonia)

• Nitrogen has 5 valence electrons.
• Each hydrogen contributes 1 valence electron.
• Nitrogen forms three sigma bonds with three hydrogen atoms.
• There is one lone pair on nitrogen.
• The structure is designated as AX₃E.
• The electron pair geometry is tetrahedral.
• The molecular geometry is trigonal pyramidal.
• The H−N−H bond angle is 107°, which is less than the ideal tetrahedral angle of 109.5°.
• This decrease is due to the lone pair-bond pair repulsion, which is stronger than bond pair-bond pair repulsion.

Two Bonding Pairs and Two Lone Pairs (AX₂E₂)

Shape: Bent (V-shaped)

• The central atom is surrounded by two bonding pairs and two lone pairs.
• There are four electron pairs in total.
• The electron pair geometry is tetrahedral.
• However, the molecular shape is determined by the positions of the atoms only.
• Since two positions are occupied by lone pairs, the shape is bent (V-shaped).
• The bond angle is less than 109.5°.

Example: H₂O (Water)

• Oxygen has 6 valence electrons.
• Each hydrogen contributes 1 valence electron.
• Oxygen forms two sigma bonds with two hydrogen atoms.
• There are two lone pairs on oxygen.
• The structure is designated as AX₂E₂.
• The electron pair geometry is tetrahedral.
• The molecular geometry is bent (V-shaped).
• The H−O−H bond angle is 104.5°, which is even less than the H−N−H angle in NH₃ (107°).
• This is because there are two lone pairs on the central atom rather than one, causing greater repulsion and a greater reduction in bond angle.

Comparison of Four Electron Pair Molecules

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.

• CH₄ (no lone pairs) has a bond angle of 109.5°.
• NH₃ (one lone pair) has a bond angle of 107°.
• H₂O (two lone pairs) has a bond angle of 104.5°.

Intermediate Level

AX₄ Molecules (Four Bonding Pairs)

• Central atom: A
• Four bonded atoms: X
• No lone pairs on A
• Shape: Tetrahedral
• Bond angle: 109.5°

• Carbon has 4 valence electrons.
• Each hydrogen contributes 1 electron for bonding.
• Total electron pairs around C: 4
• All are bonding pairs.
• Arrangement: Tetrahedral.
• Shape: Tetrahedral.

AX₃E Molecules (Three Bonding Pairs + One Lone Pair)

• Central atom: A
• Three bonded atoms: X
• One lone pair: E
• Shape: Trigonal pyramidal
• Bond angle: < 109.5°

• Nitrogen has 5 valence electrons.
• Each hydrogen contributes 1 electron for bonding.
• Three bonding pairs and one lone pair around nitrogen.
• Lone pair repels more strongly.
• Shape: Trigonal pyramidal.
• Bond angle: 107°.

AX₂E₂ Molecules (Two Bonding Pairs + Two Lone Pairs)

• Central atom: A
• Two bonded atoms: X
• Two lone pairs: E₂
• Shape: Bent (V-shaped)
• Bond angle: < 109.5°

• Oxygen has 6 valence electrons.
• Each hydrogen contributes 1 electron for bonding.
• Two bonding pairs and two lone pairs around oxygen.
• Lone pairs repel more strongly.
• Shape: Bent (V-shaped).
• Bond angle: 104.5°.

Table of Four Electron Pair Molecules

Important Points

• 1. AX₄ molecules have four bonding pairs and no lone pairs.
• 2. Shape: Tetrahedral.
• 3. Bond angle: 109.5°.
• 4. Example: CH₄, CCl₄.
• 5. AX₃E molecules have three bonding pairs and one lone pair.
• 6. Shape: Trigonal pyramidal.
• 7. Bond angle: < 109.5° (e.g., 107°).
• 8. Example: NH₃, PH₃.
• 9. AX₂E₂ molecules have two bonding pairs and two lone pairs.
• 10. Shape: Bent (V-shaped).
• 11. Bond angle: < 109.5° (e.g., 104.5°).
• 12. Example: H₂O, H₂S.
• 13. Lone pairs repel more strongly than bonding pairs.

Remember

• - AX₄: Four bonding pairs, no lone pairs
• - Shape: Tetrahedral
• - Bond angle: 109.5°
• - Example: CH₄, CCl₄
• - AX₃E: Three bonding pairs, one lone pair
• - Shape: Trigonal pyramidal
• - Bond angle: < 109.5° (e.g., 107°)
• - Example: NH₃, PH₃
• - AX₂E₂: Two bonding pairs, two lone pairs
• - Shape: Bent (V-shaped)
• - Bond angle: < 109.5° (e.g., 104.5°)
• - Example: H₂O, H₂S
• - Repulsion order: LP-LP > LP-BP > BP-BP

Common Mistakes

• 1. Thinking AX₃E is tetrahedral: The molecular shape is trigonal pyramidal because the lone pair is not counted in the molecular shape.
• 2. Thinking AX₂E₂ is tetrahedral: The molecular shape is bent because two positions are occupied by lone pairs.
• 3. Forgetting that lone pairs reduce bond angles: Lone pairs repel more strongly than bonding pairs.
• 4. Confusing electron pair geometry with molecular geometry: Electron pair geometry includes lone pairs; molecular geometry considers only atoms.
• 5. Ignoring the repulsion order: LP-LP > LP-BP > BP-BP.

Quick Revision

• AX₄ molecules have four bonding pairs and no lone pairs.
• Shape: Tetrahedral.
• Bond angle: 109.5°.
• Example: CH₄, CCl₄.
• AX₃E molecules have three bonding pairs and one lone pair.
• Shape: Trigonal pyramidal.
• Bond angle: < 109.5° (e.g., 107°).
• Example: NH₃, PH₃.
• AX₂E₂ molecules have two bonding pairs and two lone pairs.
• Shape: Bent (V-shaped).
• Bond angle: < 109.5° (e.g., 104.5°).
• Example: H₂O, H₂S.
• Lone pairs repel more strongly than bonding pairs.

60-Second Revision

• AX₄: Four bonding pairs, no lone pairs
• Shape: Tetrahedral
• Bond angle: 109.5°
• Example: CH₄, CCl₄
• AX₃E: Three bonding pairs, one lone pair
• Shape: Trigonal pyramidal
• Bond angle: < 109.5° (e.g., 107°)
• Example: NH₃, PH₃
• AX₂E₂: Two bonding pairs, two lone pairs
• Shape: Bent (V-shaped)
• Bond angle: < 109.5° (e.g., 104.5°)
• Example: H₂O, H₂S
• Repulsion order: LP-LP > LP-BP > BP-BP

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