Study Notes on Octet Rule Exceptions

Exceptions to the Octet Rule

• Learning Objectives: Draw Lewis structures for molecules with exceptions to the octet rule.

• Categories of Exceptions:

  1. Hypervalent Molecules: Central atom has more than eight electrons.

  2. Odd-Electron Molecules: Have an odd number of valence electrons, leading to unpaired electrons.

  3. Electron-Deficient Species: Central atom has fewer than eight electrons.

Hypervalent Molecules

• Elements in Period 2: (C, N, O, F) do not exceed an octet (only have four valence orbitals).

• Elements in Period 3 and higher: Can accommodate more than eight electrons due to the presence of d subshells.

• Examples:

  • $( ext{SF}_6)$ : Sulfur shares six pairs of electrons.

  • $( ext{XeF}_4)$: Central atom has lone pairs and electrons without fulfilling the octet of outer atoms.

Electron-Deficient Molecules

• Common in central atoms from groups 2 and 13. Examples include:

  • Beryllium Dihydride $( ext{BeH}_2)$: Beryllium has only 4 electrons.

  • Boron Trifluoride $( ext{BF}_3)$: Boron has 6 electrons, can form single bonds but experimental evidence shows odd bond lengths, indicating reactivity due to electron deficiency.

Odd-Electron Molecules

• Definition: Molecules with odd number of valence electrons termed free radicals.

• Example:

  • Nitric Oxide $( ext{NO})$: A biological signaling molecule, exists as a free radical.

General Guidelines for Drawing Lewis Structures

1. Most elements (except H) surround themselves with 8 electrons.

2. If multiple structures exist, check formal charges; prefer fewer lone pairs.

3. For identical formal charges, any structure can be chosen; resonance may exist.

4. Period 3 and higher can exceed octet.

5. Groups 1, 2, and 13 can have less than an octet.