One important exception to the Octet Rule is that some elements can have Lewis Structures with expanded octets. Elements in Period Three and below on the Periodic Table are capable of having expanded octets where they have more than eight valance electrons. They don't always have expanded octets, but it is a possibility.

Expanded octets are possible in Lewis Structures because the elements in Period Three and below have d orbitals that can take part in bonding. Two common elements that can have more than eight valence electrons are Sulfur (S) and Phosphorus (P). For example, the compounds SF6 and Pl5 both have expanded octets. For other compounds, like SO4 2-, the Sulfate ion, it is less obvious if there will be an expanded octet just by drawing the Lewis Structure.

Knowledge of these octet exceptions is important for drawing Lewis Structures. When drawing Lewis dot Structures that involve elements which may be exceptions to the octet rule, you can calculate the formal charges to be sure you have the best/most likely Lewis Structure for the compound.


--- More Information ---
• How to Draw Lewis Structures: [ Ссылка ]
• Formal Charges: [ Ссылка ]
• Finding Valence Electrons (molecule): [ Ссылка ]
• Duet Rule: [ Ссылка ]
• My Chemical Bonding Webpage: [ Ссылка ]


---Steps to Write Lewis Structures---
1. Find the total valence electrons for the molecule.
2. Put the least electronegative atom in the center. Note: Hydrogen (H) always goes outside.
3. Put two electrons between atoms to form a chemical bond.
4. Complete octets on outside atoms.
5. If central atom does not have an octet, move electrons from outer atoms to form double or triple bonds.
6. If the element is in Period Three of below be sure to check formal charges to be sure you have the best Lewis Structure.


Lewis Structures are important to learn because they help us understand how atoms and electrons are arranged in a molecule. This can help us determine the molecular geometry, how the molecule might react with other molecules, and some of the physical properties of the molecule (like boiling point and surface tension).

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