Octet expansion occurs when an atom in a molecule or ion has more than eight electrons in its valence shell. This is possible for elements in the third period and beyond, as they have access to d orbitals that can accommodate additional electrons. Here are some examples of compounds that exhibit octet expansion:

Common Examples:

* Phosphorus pentachloride (PCl5): Phosphorus (P) has 5 valence electrons and forms 5 bonds with chlorine (Cl) atoms, resulting in 10 electrons around P.

* Sulfur hexafluoride (SF6): Sulfur (S) has 6 valence electrons and forms 6 bonds with fluorine (F) atoms, resulting in 12 electrons around S.

* Xenon tetrafluoride (XeF4): Xenon (Xe) has 8 valence electrons and forms 4 bonds with fluorine (F) atoms, resulting in 12 electrons around Xe.

* Iodine pentafluoride (IF5): Iodine (I) has 7 valence electrons and forms 5 bonds with fluorine (F) atoms, resulting in 12 electrons around I.

* Phosphoric acid (H3PO4): The phosphorus atom has 10 electrons in its valence shell.

Factors influencing octet expansion:

* Electronegativity difference: Elements with a higher electronegativity difference tend to form compounds with octet expansion.

* Size of the central atom: Larger central atoms are more likely to undergo octet expansion because they have more available d orbitals.

* Bonding: Multiple bonds (like double or triple bonds) can also contribute to octet expansion.

Note: Octet expansion is not the only way for elements in the third period and beyond to achieve stability. They can also form compounds with less than eight electrons in their valence shell, as seen in compounds like boron trifluoride (BF3).

If you'd like more specific examples or have any further questions about octet expansion, feel free to ask!