Elements in period 3 and beyond may hold more than eight valence electrons, as in SF6 and PCl5.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Elements of period 3 and below can hold more than eight valence electrons around a central atom, a situation called an expanded octet. It is common when the central atom is large and surrounded by electronegative ligands such as fluorine or chlorine. Historically the extra electrons were said to occupy empty d orbitals, but modern calculations show that d orbital participation is small. Bonding is better described by ionic contributions and three-center four-electron bonds. Expanded octets are seen in species such as PCl5, SF6 and the sulfate ion.
Key ideas: Expect an expanded octet only for period 3 or lower atoms bonded to small electronegative ligands: F, O, Cl. Count electron domains instead of electrons: five domains for PCl5, six for SF6 and XeF4. Typical cases are SF4, PF5, ClF3, XeF2, SF6, ICl4-. Prefer a structure with an octet if formal charges are acceptable.
Common mistakes and connections: Mistakes include expanding the octet of C, N or O, which is impossible in the second period, and expanding it where an octet structure with a formal charge is better, as for sulfate and phosphate in current practice. Connects to hypervalency, VSEPR with five and six domains and sp3d labels.
Sulfur in sulfate is a period 3 atom that can carry more than eight electrons, allowing four S-O bonds, some with double-bond character, around it. The interactive lesson shows this as a live animation or chart.
In SF6 the central sulfur atom is surrounded by twelve electrons in six bonding pairs.
Elements in period 3 and beyond may hold more than eight valence electrons, as in SF6 and PCl5.
Expanded Octet is a intermediate topic in Bonding & Molecular Structure. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Bonding & Molecular Structure first: Formal Charge, Resonance, Resonance Hybrid.
The next topics in Bonding & Molecular Structure are Incomplete Octet and Odd-Electron Molecules.
Yes. The interactive lesson includes a self-check question on Expanded Octet, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Expanded Octet is topic 14 of 104 in Bonding & Molecular Structure.