3.5.4 Shapes of Molecules Containing Five Electron Pairs

This topic explains the shape of molecules where the central atom is surrounded by five electron pairs and no lone pairs.

Definition

Five Electron Pairs (AX₅): A molecule in which the central atom is surrounded by five bonding pairs of electrons and no lone pairs.

Main Idea

When a central atom is surrounded by five electron pairs and no lone pairs, the electron pairs arrange themselves as far apart as possible to minimize repulsion. This gives the molecule a trigonal bipyramidal shape. The bond angles are 90° and 120°. Some molecules with five electron pairs have expanded octets because the central atom has more than 8 electrons around it.

Core Concept

• Three equatorial positions at 120° to each other.
• Two axial positions at 90° to the equatorial plane.

The general formula for such molecules is AX₅, where A is the central atom and X represents the bonded atoms.

Detailed Explanation

Shape: Trigonal Bipyramidal

• The central atom is surrounded by five bonding pairs.
• There are no lone pairs on the central atom.
• To minimize repulsion, the five electron pairs are arranged in a trigonal bipyramidal geometry.
• Three chlorine atoms lie in the same plane as the central atom (equatorial positions).
• The bond angles within this plane are 120°.
• Two chlorine atoms lie above and below this plane at 90° to it (axial positions).

Example: PCl₅ (Phosphorus Pentachloride)

• Phosphorus (P) has atomic number 15.
• Ground state configuration: 1s² 2s² 2p⁶ 3s² 3p³

• Phosphorus has 5 valence electrons.
• Each chlorine atom contributes 1 valence electron for bonding.
• Phosphorus forms five sigma bonds with five chlorine atoms.
• The Lewis structure: P with five Cl atoms around it.

• There are five bonding pairs around the central phosphorus atom.
• There are no lone pairs.
• The arrangement that minimizes repulsion is trigonal bipyramidal.
• Three chlorine atoms lie in a plane at 120° to each other.
• Two chlorine atoms lie above and below the plane at 90°.
• The shape is trigonal bipyramidal.

General Formula: AX₅

Other Examples

Expanded Octet / Hypervalency

• The octet rule applies well to atoms in the second row of the periodic table (e.g., C, N, O, F).
• Elements in the third and fourth rows can have expanded octets because they have available d, f, and g subshells.
• In PCl₅, phosphorus has 10 electrons around it (5 bonding pairs), which is more than the octet (8 electrons).
• This is called an expanded octet or hypervalency.

Working Out the Shape of PCl₅

• Phosphorus: 5 valence electrons
• Each chlorine: 7 valence electrons (but only 1 is used for bonding)
• Total electrons around P: 5 (from P) + 5 (from Cl) = 10 electrons = 5 pairs

• 5 bonding pairs
• 0 lone pairs

3. Arrange electron pairs: - To minimize repulsion, the 5 pairs arrange in a trigonal bipyramidal geometry.

4. Determine molecular shape: - Since there are no lone pairs, the molecular shape is also trigonal bipyramidal.

Intermediate Level

AX₅ Molecules (Five Bonding Pairs)

• Central atom: A
• Five bonded atoms: X
• No lone pairs on A
• Shape: Trigonal bipyramidal
• Bond angles: 90° and 120°

• Phosphorus has 5 valence electrons.
• Each chlorine contributes 1 electron for bonding.
• Total electron pairs around P: 5
• All are bonding pairs.
• Arrangement: Trigonal bipyramidal.
• Three Cl atoms in equatorial positions at 120°.
• Two Cl atoms in axial positions at 90°.
• Shape: Trigonal bipyramidal.

Table of AX₅ Molecules

Expanded Octet

• Elements in the third period and beyond can have more than 8 electrons.
• In PCl₅, phosphorus has 10 electrons around it.
• This is possible because phosphorus has available d orbitals in the third shell.

Important Points

• 1. AX₅ molecules have five bonding pairs and no lone pairs.
• 2. Shape: Trigonal bipyramidal.
• 3. Bond angles: 90° and 120°.
• 4. Example: PCl₅, PF₅, SbCl₅.
• 5. Three atoms in equatorial positions (120° apart).
• 6. Two atoms in axial positions (90° to the equatorial plane).
• 7. Phosphorus undergoes sp³d hybridization in PCl₅.
• 8. PCl₅ has an expanded octet (10 electrons around P).

Remember

• - AX₅: Five bonding pairs, no lone pairs
• - Shape: Trigonal bipyramidal
• - Bond angles: 90°, 120°
• - Example: PCl₅, PF₅
• - Hybridization: sp³d
• - Equatorial positions: 120°
• - Axial positions: 90°
• - Expanded octet: P has 10 electrons

Common Mistakes

• 1. Thinking all five bonds are equivalent: The equatorial and axial bonds are not equivalent.
• 2. Forgetting the bond angles: Equatorial = 120°; Axial = 90°.
• 3. Ignoring the expanded octet: PCl₅ has 10 electrons around phosphorus.
• 4. Confusing electron pair geometry with molecular geometry: For AX₅, both are trigonal bipyramidal (since there are no lone pairs).

Quick Revision

• AX₅ molecules have five bonding pairs and no lone pairs.
• Shape: Trigonal bipyramidal.
• Bond angles: 90° and 120°.
• Example: PCl₅, PF₅, SbCl₅.
• Three atoms in equatorial positions (120° apart).
• Two atoms in axial positions (90° to the equatorial plane).
• Phosphorus undergoes sp³d hybridization in PCl₅.
• PCl₅ has an expanded octet (10 electrons around P).

60-Second Revision

• AX₅: Five bonding pairs, no lone pairs
• Shape: Trigonal bipyramidal
• Bond angles: 90°, 120°
• Example: PCl₅, PF₅
• Hybridization: sp³d
• Equatorial positions: 120°
• Axial positions: 90°
• Expanded octet: P has 10 electrons

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