3.5.5 Shapes of Molecules Containing Six Electron Pairs

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

Definition

Six Electron Pairs (AX₆): A molecule in which the central atom is surrounded by six bonding pairs of electrons and no lone pairs.

Main Idea

When a central atom is surrounded by six electron pairs and no lone pairs, the electron pairs arrange themselves as far apart as possible to minimize repulsion. This gives the molecule an octahedral shape. All bond angles are 90°. Molecules with six electron pairs have expanded octets because the central atom has more than 8 electrons around it.

Core Concept

• Six positions at 90° to each other.
• All positions are equivalent.
• The general formula for such molecules is AX₆, where A is the central atom and X represents the bonded atoms.

Detailed Explanation

Shape: Octahedral

• The central atom is surrounded by six bonding pairs.
• There are no lone pairs on the central atom.
• To minimize repulsion, the six electron pairs are arranged in an octahedral geometry.
• All bond angles are 90°.
• The shape of the molecule is octahedral.

Example: SF₆ (Sulfur Hexafluoride)

• Sulfur (S) has atomic number 16.
• Ground state configuration: 1s² 2s² 2p⁶ 3s² 3p⁴

• Sulfur has 6 valence electrons.
• Each fluorine atom contributes 1 valence electron for bonding.
• Sulfur forms six sigma bonds with six fluorine atoms.
• The Lewis structure: S with six F atoms around it.

• There are six bonding pairs around the central sulfur atom.
• There are no lone pairs.
• The arrangement that minimizes repulsion is octahedral.
• All bond angles are 90°.
• The shape is octahedral.

General Formula: AX₆

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 SF₆, sulfur has 12 electrons around it (6 bonding pairs), which is more than the octet (8 electrons).
• This is called an expanded octet or hypervalency.

Working Out the Shape of SF₆

• Sulfur: 6 valence electrons
• Each fluorine: 7 valence electrons (but only 1 is used for bonding)
• Total electrons around S: 6 (from S) + 6 (from F) = 12 electrons = 6 pairs

• 6 bonding pairs
• 0 lone pairs

3. Arrange electron pairs: - To minimize repulsion, the 6 pairs arrange in an octahedral geometry.

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

Intermediate Level

AX₆ Molecules (Six Bonding Pairs)

• Central atom: A
• Six bonded atoms: X
• No lone pairs on A
• Shape: Octahedral
• Bond angle: 90°

• Sulfur has 6 valence electrons.
• Each fluorine contributes 1 electron for bonding.
• Total electron pairs around S: 6
• All are bonding pairs.
• Arrangement: Octahedral.
• All bond angles are 90°.
• Shape: Octahedral.

Table of AX₆ Molecules

Expanded Octet

• Elements in the third period and beyond can have more than 8 electrons.
• In SF₆, sulfur has 12 electrons around it.
• This is possible because sulfur has available d orbitals in the third shell.

Important Points

• 1. AX₆ molecules have six bonding pairs and no lone pairs.
• 2. Shape: Octahedral.
• 3. Bond angle: 90°.
• 4. Example: SF₆, SeF₆, PF₆⁻, SiF₆²⁻.
• 5. All six positions are equivalent.
• 6. Sulfur undergoes sp³d² hybridization in SF₆.
• 7. SF₆ has an expanded octet (12 electrons around S).

Remember

• - AX₆: Six bonding pairs, no lone pairs
• - Shape: Octahedral
• - Bond angle: 90°
• - Example: SF₆, SeF₆
• - Hybridization: sp³d²
• - Expanded octet: S has 12 electrons

Common Mistakes

• 1. Thinking all six bonds are not equivalent: In an octahedral geometry, all six positions are equivalent.
• 2. Forgetting the bond angle: All bond angles are 90°.
• 3. Ignoring the expanded octet: SF₆ has 12 electrons around sulfur.
• 4. Confusing electron pair geometry with molecular geometry: For AX₆, both are octahedral (since there are no lone pairs).

Quick Revision

• AX₆ molecules have six bonding pairs and no lone pairs.
• Shape: Octahedral.
• Bond angle: 90°.
• Example: SF₆, SeF₆, PF₆⁻, SiF₆²⁻.
• All six positions are equivalent.
• Sulfur undergoes sp³d² hybridization in SF₆.
• SF₆ has an expanded octet (12 electrons around S).

60-Second Revision

• AX₆: Six bonding pairs, no lone pairs
• Shape: Octahedral
• Bond angle: 90°
• Example: SF₆, SeF₆
• Hybridization: sp³d²
• Expanded octet: S has 12 electrons

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