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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S2.2 - The covalent model

2.2.1 Covalent Bonds and Lewis Formulas 2.2.2 Bond Types 2.2.3 Co-coordination (Dative) Bonds 2.2.4 VSEPR Shapes of Molecules 2.2.5 Electronegativity and Bond Polarity 2.2.6 Polarity and Dipole Moments 2.2.7 Covalent Network Structures and Allotropes 2.2.8 Intermolecular Forces 2.2.9 Physical Properties of Covalent Substances 2.2.10 Chromatography and Intermolecular Forces 2.2.11 Resonance Structures (AHL) 2.2.12 Benzene and Resonance (AHL) 2.2.13 Expanded Octet and VSEPR (AHL) 2.2.14 Formal Charge (AHL) 2.2.15 Sigma and Pi Bonds (AHL) 2.2.16 Hybridization (AHL)

Expanded Octet and VSEPR Geometry HL Only

Specification Reference S2.2.13

Quick Notes

  • Some atoms (typically in period 3 or higher) can have more than 8 electrons in their valence shell – this is called an expanded octet.
  • These species can have 5 or 6 electron domains around the central atom.
  • We can still use VSEPR theory to deduce:
    • Electron domain geometry (arrangement of all regions of electron density)
    • Molecular geometry (actual shape based on bonded atoms only)
  • Common geometries include:
    • 5 domains → trigonal bipyramidal (e.g. PCl5)
    • 6 domains → octahedral (e.g. SF6)

Full Notes

What Is an Expanded Octet?

The octet rule limits atoms to 8 electrons in their outer shell. However, atoms in period 3 and beyond have d orbitals available and can accommodate more than 8 electrons.

These atoms can form molecules with 5 or 6 electron domains around the central atom, giving an expanded octet.

For Example:

IB Chemistry diagram showing atoms such as sulfur expanding their octet beyond 8 electrons.

VSEPR Predictions for 5 and 6 Electron Domains

We can use the Valence Shell Electron Pair Repulsion (VSEPR see S2.2.4) model to predict:

Species with 5 Electron Domains (Trigonal Bipyramidal Base Geometry)

IB Chemistry VSEPR diagram showing trigonal bipyramidal arrangement with 5 electron domains.
Electron Domains Electron Domain Geometry Molecular Geometry Example
5 bonding pairs Trigonal bipyramidal Trigonal bipyramidal PCl5
4 bonding + 1 lone pair Trigonal bipyramidal See-saw SF4
3 bonding + 2 lone pairs Trigonal bipyramidal T-shaped ClF3
2 bonding + 3 lone pairs Trigonal bipyramidal Linear XeF2

Species with 6 Electron Domains (Octahedral Base Geometry)

IB Chemistry VSEPR diagram showing octahedral arrangement with 6 electron domains.
Electron Domains Electron Domain Geometry Molecular Geometry Example
6 bonding pairs Octahedral Octahedral SF6
5 bonding + 1 lone pair Octahedral Square pyramidal BrF5
4 bonding + 2 lone pairs Octahedral Square planar XeF4

Drawing Lewis Structures and Visualizing Shape

When drawing Lewis structures for expanded octets:

Use dash-and-wedge diagrams to show 3D shape when helpful:

Summary