Core Concept

Definition: 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.

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.