Illustrated Glossary of Organic Chemistry

Expanded octet (hypervalent): A valence shell electron count that exceeds eight electrons.

In methane (CH₄), carbon has a full octet (eight valence electrons).


In phosphate ion (PO₄^3-), each oxygen has a full octet (eight valence electrons), whereas phosphorus has an expanded octet (ten valence electrons).


In sulfuric acid (H₂SO₄), each oxygen has a full octet (eight valence electrons), whereas sulfur has an expanded octet (twelve valence electrons).