Expanded Octet

Core Concept

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a full set of 8 valence electrons, mimicking the electron configuration of noble gases.

Limitations of the Octet Rule: Not all elements adhere to this rule, especially those in and beyond the third period of the periodic table.

An expanded octet occurs when atoms in the third period or beyond can have more than eight electrons in their valence shell. This happens due to the availability of d-orbitals for bonding.

Practice Tips

Identify the Period: If the central atom is in Period 1 or 2 (like Carbon or Nitrogen), it never expands its octet; only atoms from Period 3 and below have this capability.
Trust Your Math: When your total valence electron count is an odd number, don't panic; just place the remaining single electron on the most appropriate atom (usually the less electronegative one).
Count the Neighbors: If a central atom is bonded to five or six other atoms (like in $PCl_5$), it is a guaranteed expanded octet because each bond represents two electrons.
Check Formal Charges: Always calculate formal charges for expanded octets to verify if extra double bonds are actually necessary to stabilize the molecule.

Test Yourself

Assorted Multiple Choice

A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Lewis Structures - Expanded Octet

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Core Concept

What is an Expanded Octet?

An expanded octet occurs when atoms in the third period or beyond can have more than eight electrons in their valence shell. This happens due to the availability of d-orbitals for bonding.

Why Can Some Atoms Expand Their Octet?

Explanation: Atoms in the third period and beyond have access to the 3d subshell, which allows them to accommodate extra electrons.
Energy Considerations: For these atoms, the energy required to use the 3d orbitals is low enough that they can expand their octet to bond with more atoms or accept extra lone pairs.

Examples of Molecules with Expanded Octets

Phosphorus Pentachloride (PCl₅)
- Structure: Phosphorus forms 5 covalent bonds with chlorine, giving it 10 electrons around its central atom.
- Lewis Structure: [Insert drawing space]
Sulfur Hexafluoride (SF₆)
- Structure: Sulfur forms 6 covalent bonds with fluorine, giving it 12 electrons around the central atom.
- Lewis Structure: [Insert drawing space]
Xenon Tetrafluoride (XeF₄)
- Structure: Xenon forms 4 covalent bonds with fluorine, giving it 12 valence electrons while maintaining a square planar shape.
- Lewis Structure: [Insert drawing space]

5. When and How to Recognize an Expanded Octet

Clues:
1. The central atom is from the third period or beyond.
2. The molecule requires more than 4 bonding pairs of electrons.
3. There are more than 8 electrons around the central atom in the Lewis structure.

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