Structure 2.2.13 – Expanded Octet and VSEPR Geometry (5 & 6 Domains)

Atoms from Period 3 and beyond (e.g. phosphorus, sulfur, chlorine, etc.) have access to empty d orbitals and can therefore accommodate more than 8 electrons in their valence shell. This leads to the formation of molecules with expanded octets. These molecules often involve five or six electron domains around the central atom.

Key Principle (VSEPR Model):

Electron pairs (bonding and non-bonding) repel each other and arrange themselves around the central atom to minimize repulsion. The resulting 3D shape depends on the number of domains and whether they are bonding or lone pairs.

Five Electron Domains:

• Electron Domain Geometry: Trigonal bipyramidal
• Bond Angles: 90° and 120°

Six Electron Domains:

• Electron Domain Geometry: Octahedral
• Bond Angles: 90°

Visualizing Expanded Octet Structures:

Use Lewis structures showing all bonding and lone pairs. Then apply the VSEPR model to determine the geometry by minimizing repulsion between domains.

Important Notes:

• Only atoms in Period 3 and beyond can expand the octet.
• Central atoms in these species often have more than four bonded atoms or lone pairs.
• Lone pairs take up more space than bonding pairs and distort bond angles.