Limitations of Lewis Structures (College Board AP Chemistry)

• There are some chemical compounds that cannot be represented with Lewis diagrams accurately
• This occurs because the elements involved do not follow the octet rule

Molecules with odd number of electrons

• One of the most common cases occurs when there is an odd number of electrons in the molecule
• There are actually few stable molecules in nature with odd number of electrons, because an molecules with unpaired electrons are highly reactive and unstable
• These odd electron species are called free radicals

Examples of Free Radicals

Molecules with an odd number of electrons

Molecules with expanded valence shells

• There are some molecules that exceed the eight electrons in their valence shell
• An atom with an expanded octet can hold up to ten electrons, or even twelve
• These expansion occurs because there are empty f-orbitals available for bonding after the third energy shell
  • Therefore, the third period elements can occasionally exceed the octet to accommodate additional electrons
• Expanded octets occurs occasionally when a non metal central atom from the third period or below, is surrounded by terminal atoms with high electronegativity values
• The most common atoms that form expanded octets are: S and P

The PCl₅ molecule

 

The octet of the central phosphorus atom has been expanded to hold 10 electrons

The SF₆ molecule

The octet of the central sulfur atom has been expanded to hold 12 electrons