Expanded octets are a phenomenon observed in certain elements due to the presence of d-orbitals in their valence shells that are available for bonding.

To elaborate, the octet rule posits that atoms tend to bond in such a manner that each achieves a total of eight electrons in their valence shells, thereby attaining a noble gas-like electronic configuration. This guideline is primarily based on the behavior of main group elements, which typically form bonds to achieve an octet. However, it is important to note that the octet rule is not universally applicable, and exceptions do exist.

One notable exception is the occurrence of expanded octets. This term describes the situation where an atom possesses more than eight electrons in its valence shell. Elements from the third period and beyond are capable of this due to their availability of d-orbitals for bonding. These d-orbitals can hold up to ten electrons, enabling these elements to form more than four bonds and, consequently, to exceed the octet.

For instance, phosphorus ($P$), sulfur ($S$), and chlorine ($Cl$) are common elements that exhibit expanded octets. Phosphorus can form five bonds in the molecule phosphorus pentachloride ($PCl_5$), while sulfur can form six bonds in sulfur hexafluoride ($SF_6$). In these cases, the central atoms ($P$ and $S$) each possess ten and twelve electrons in their valence shells, respectively, surpassing the typical octet.

The ability to accommodate expanded octets significantly enhances the bonding possibilities and molecular structures of these elements. This phenomenon also allows for the formation of compounds with higher coordination numbers, which refers to the number of atoms or ions directly surrounding a central atom in a complex or coordination compound.

In conclusion, expanded octets are characteristic of certain elements that possess available d-orbitals in their valence shells for bonding. This allows them to hold more than eight electrons and to form more than four bonds, marking a significant deviation from the octet rule. Understanding expanded octets is crucial for comprehending the bonding and molecular structures of these elements.