3.5.6 Expanded Octet / Hypervalency

This topic explains the concept of expanded octet (hypervalency), why some elements can have more than eight electrons in their valence shell, and which elements can exhibit this behaviour.

Definition

Expanded Octet (Hypervalency): The phenomenon in which an atom in a molecule has more than eight electrons in its valence shell.

Hypervalent Molecule: A molecule in which the central atom has more than eight electrons in its valence shell.

Main Idea

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight electrons in their valence shell. However, this rule applies well only to atoms in the second row of the periodic table (e.g., C, N, O, F). Elements in the third and fourth rows (e.g., P, S, Cl, Se, Br) can have more than eight electrons in their valence shell because they have available d, f, and g subshells. This is called an expanded octet or hypervalency.

Core Concept

• They have available d orbitals in the same shell.
• They have larger atomic sizes, which can accommodate more electrons.
• They can form more than four bonds (e.g., PCl₅ has 5 bonds, SF₆ has 6 bonds).

Detailed Explanation

The Octet Rule and Its Limits

• Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight electrons in their valence shell.
• Applies well to atoms in the second row of the periodic table:
• Carbon (C): 1s² 2s² 2p²
• Nitrogen (N): 1s² 2s² 2p³
• Oxygen (O): 1s² 2s² 2p⁴
• Fluorine (F): 1s² 2s² 2p⁵
• These elements have only s and p orbitals available for bonding.
• They cannot accommodate more than 8 electrons in their valence shell.

• Even elements in the third and fourth rows are known to follow this rule sometimes, but not always.
• In larger atoms of the 3rd period onwards, the valence shell contains additional subshells: d, f, and g subshells.
• These additional subshells can accommodate more electrons.

What is Expanded Octet?

• When an atom has more than eight electrons in its valence shell, it is said to have an expanded octet.
• This is also called hypervalency.
• The atom uses its d orbitals (and sometimes f orbitals) to accommodate the extra electrons.

Examples of Expanded Octet

• Phosphorus has 5 valence electrons.
• It forms 5 bonds with 5 chlorine atoms.
• Total electrons around P: 10 electrons (5 bonding pairs).
• Phosphorus has an expanded octet.
• Hybridization: sp³d

• Sulfur has 6 valence electrons.
• It forms 6 bonds with 6 fluorine atoms.
• Total electrons around S: 12 electrons (6 bonding pairs).
• Sulfur has an expanded octet.
• Hybridization: sp³d²

• Sulfur has 6 valence electrons.
• It forms 4 bonds with 4 fluorine atoms.
• There is 1 lone pair on sulfur.
• Total electrons around S: 10 electrons (4 bonding pairs + 1 lone pair).
• Sulfur has an expanded octet.
• Hybridization: sp³d

• Chlorine has 7 valence electrons.
• It forms 3 bonds with 3 fluorine atoms.
• There are 2 lone pairs on chlorine.
• Total electrons around Cl: 10 electrons (3 bonding pairs + 2 lone pairs).
• Chlorine has an expanded octet.
• Hybridization: sp³d

• Xenon has 8 valence electrons.
• It forms 4 bonds with 4 fluorine atoms.
• There are 2 lone pairs on xenon.
• Total electrons around Xe: 12 electrons (4 bonding pairs + 2 lone pairs).
• Xenon has an expanded octet.
• Hybridization: sp³d²

Summary Table

Why Can Some Elements Expand Their Octet?

• Elements in the third period and beyond have d orbitals in their valence shell.
• These d orbitals can accommodate extra electrons.
• Example: Phosphorus (3rd period) has 3d orbitals available; Nitrogen (2nd period) does not.

• Larger atoms can accommodate more electrons around the nucleus.
• The valence shell is farther from the nucleus, so there is more space.

• Elements with lower electronegativity are more willing to share electrons.
• They can form more bonds.

Which Elements Can Expand Their Octet?

Note: The ability to expand the octet increases as you go down a group because the atoms get larger and have more available d orbitals.

Intermediate Level

The Octet Rule

• Atoms tend to achieve 8 electrons in their valence shell.
• Applies well to second-period elements (C, N, O, F).

Expanded Octet (Hypervalency)

• Some atoms can have more than 8 electrons in their valence shell.
• This is possible for elements in the third period and beyond.
• Reason: They have available d orbitals.

Examples

Why Can't Second-Period Elements Expand Their Octet?

• Second-period elements (C, N, O, F) have only s and p orbitals available.
• They have no d orbitals in their valence shell.
• Therefore, they cannot accommodate more than 8 electrons.

Important Points

• 1. Expanded octet = more than 8 electrons in the valence shell.
• 2. Possible for elements in the third period and beyond.
• 3. Reason: Available d orbitals.
• 4. Examples: PCl₅ (10 e⁻), SF₆ (12 e⁻), SF₄ (10 e⁻), ClF₃ (10 e⁻), XeF₄ (12 e⁻).
• 5. Hybridization: sp³d, sp³d².
• 6. Second-period elements (C, N, O, F) cannot expand their octet.
• 7. The ability to expand the octet increases down a group.

Remember

• - Expanded octet: More than 8 electrons
• - Possible for: Third period and beyond
• - Reason: Available d orbitals
• - Examples: PCl₅ (10 e⁻), SF₆ (12 e⁻)
• - Hybridization: sp³d, sp³d²
• - Second-period elements: Cannot expand octet

Common Mistakes

• 1. Thinking all atoms follow the octet rule: Some atoms have expanded octets.
• 2. Thinking second-period elements can expand their octet: They cannot (no d orbitals).
• 3. Confusing expanded octet with incomplete octet: Expanded = more than 8; Incomplete = fewer than 8.
• 4. Forgetting the role of d orbitals: d orbitals are essential for expanded octets.

Quick Revision

• Expanded octet = more than 8 electrons in the valence shell.
• Possible for elements in the third period and beyond.
• Reason: Available d orbitals.
• Examples: PCl₅ (10 e⁻), SF₆ (12 e⁻), SF₄ (10 e⁻), ClF₃ (10 e⁻), XeF₄ (12 e⁻).
• Hybridization: sp³d, sp³d².
• Second-period elements (C, N, O, F) cannot expand their octet.
• The ability to expand the octet increases down a group.

60-Second Revision

• Expanded octet: More than 8 electrons
• Possible for: Third period and beyond
• Reason: Available d orbitals
• Examples: PCl₅ (10 e⁻), SF₆ (12 e⁻)
• Hybridization: sp³d, sp³d²
• Second-period elements: Cannot expand octet

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