Summary:
- Exceptions to the octet rule fall into one of three categories: (1) an incomplete octet, (2) odd-electron molecules, and (3) an expanded octet.
- In some compounds, the number of electrons surrounding the central atom in a stable molecule is fewer than eight.
- Beryllium hydride (BeH2) consists of two single covalent bonds between Be and H (see Figure below) In phosphorus pentachloride, the central phosphorus atom makes five single bonds to chlorine atoms and as a result has ten electrons surrounding it.