An expanded octet occurs when an atom has more than eight electrons around it in a Lewis structure. In IB Chemistry, this is Additional Higher Level content within Structure 2.2, the covalent model.
How Does an Expanded Octet Form?
When counting electrons around an atom, each bonding pair and lone pair contributes two electrons. Some central atoms from period 3 or later can form stable structures containing five or six electron domains, so more than eight electrons may surround them.
For example, phosphorus forms five single bonds in . Each bond contains two electrons, giving phosphorus electrons around it. Similarly, sulfur has six bonding pairs in , giving it surrounding electrons.
| Molecule | Electrons around the central atom | VSEPR geometry |
|---|---|---|
| 10 electrons from five bonding pairs | Trigonal bipyramidal | |
| 12 electrons from six bonding pairs | Octahedral | |
An expanded octet is therefore closely connected to VSEPR theory: five electron domains produce a trigonal bipyramidal electron-domain geometry, while six produce an octahedral electron-domain geometry.
A common misconception is that every central atom can expand its octet. Period 2 elements such as carbon, nitrogen, oxygen and fluorine cannot exceed eight electrons in IB Lewis structures. Also, an expanded octet does not mean that the atom has gained electrons to form an ion; bonding electrons are shared and counted around both bonded atoms.
Exam Technique
For an IB question, draw the complete Lewis structure, include all lone pairs, and count the electrons around the central atom. If there are more than eight, identify an expanded octet, then use the total number of electron domains to deduce the molecular geometry.
