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.