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General Chemistry I · Chemical Bonding & Molecular Geometry

Octet Rule Exceptions

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

The octet rule is a powerful guideline, not an absolute law. Three classes of molecules violate it: (1) electron-deficient species in which the central atom has fewer than eight electrons (BF₃, BeCl₂); (2) odd-electron species (free radicals) with an unpaired electron (NO, NO₂); and (3) expanded-octet species in which a period-3 or heavier central atom holds more than eight electrons using its vacant d orbitals (SF₆, PCl₅).

Why this matters

Octet exceptions are not obscure trivia — they explain the behavior of some of chemistry's most important species. BF₃'s electron deficiency makes it a classic Lewis-acid catalyst. NO is a reactive free radical that acts as a signaling molecule in the body (vasodilation) and a pollutant from combustion. SF₆ is a potent electrical insulator and one of the most potent greenhouse gases. Recognizing when the octet rule breaks down is essential for predicting reactivity and properties.

The college version

Key Ideas

  • Incomplete (electron-deficient) octet: central atoms like B (3 valence e⁻) and Be (2 valence e⁻) can be stable with 6 or 4 electrons. Example: BF₃ has B with only 6 electrons.
  • Odd-electron species (free radicals): a molecule with an odd number of valence electrons must have at least one unpaired electron. Example: NO has 11 electrons; NO₂ has 17.
  • Expanded octet (hypervalent): central atoms in period 3 or below (P, S, Cl, Xe…) can exceed 8 electrons by using empty 3d (or higher) orbitals. Examples: SF₆ (12 e⁻ around S), PCl₅ (10 e⁻ around P).
  • Why period 3+ only: second-period elements (C, N, O, F) have no accessible d orbitals and cannot exceed 8 valence electrons.
  • Electron-deficient atoms act as Lewis acids (electron-pair acceptors) — e.g., BF₃ readily accepts a lone pair to complete its octet.

Equations and Variables

  • Total valence electrons for odd-electron species simply comes out odd:
    • NO: N (5) + O (6) = 11 electrons → one unpaired electron.
    • NO₂: N (5) + 2 × O (6) = 17 electrons.
  • No new equation is needed; the "exception" is recognized by counting electrons (odd), or by a central atom that is short (B, Be) or overfilled (period-3+ atom with >8).

How It Works

  1. Count valence electrons and draw the skeleton as usual.
  2. If the total is odd: at least one atom must carry an unpaired electron. Place it to minimize formal charge (often on the less electronegative atom). Example: NO is best drawn N=O with the unpaired electron on N.
  3. If a small central atom is short of an octet and no lone pairs are available to share: accept the incomplete octet (B in BF₃, Be in BeCl₂). Do not invent extra bonds.
  4. If the central atom is period 3 or heavier and the terminal atoms have no lone pairs left but the count demands more bonds: place extra electron pairs on the central atom to reach 10 or 12 electrons (PCl₅, SF₆).

Worked Example

Draw Lewis structures for BF₃, NO, and SF₆.

  1. BF₃ (24 valence e⁻): B is central with three single B–F bonds (6 e⁻), and each F carries three lone pairs (18 e⁻). Boron has only 6 electrons — an incomplete octet. (Boron has no lone pair to share, and it cannot form a fourth bond without exceeding its 3 valence electrons.)
  1. NO (11 valence e⁻): N and O share a double bond (N=O), O gets two lone pairs, N gets one lone pair plus one unpaired electron. The unpaired electron sits on nitrogen (giving N a FC of 0 and O a FC of 0). NO is a free radical.
  1. SF₆ (48 valence e⁻): S is central with six single S–F bonds (12 e⁻), and each F carries three lone pairs (36 e⁻). Sulfur holds 12 electrons — an expanded octet, possible only because sulfur is in period 3 and has empty 3d orbitals.

Common Confusions

  • "The octet rule is never broken." — Wrong: three whole categories of exceptions exist; it is a guideline, not a law.
  • "Any central atom can have an expanded octet." — Wrong: only period-3 and heavier atoms can, because they have accessible d orbitals. Carbon, nitrogen, oxygen, and fluorine cannot exceed 8 electrons.
  • "BF₃ should be drawn with a B=F double bond to give B an octet." — Wrong: that would exceed boron's 3 valence electrons; the correct structure has an incomplete (6-electron) octet.
  • "Odd-electron molecules violate charge neutrality." — Wrong: they are neutral but have one unpaired electron (a free radical).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The octet rule is like a "seatbelt rule" that most atoms follow — but a few cars are exempt. Boron's car only has room for three passengers (six electrons) and is perfectly happy underfilled. Some cars carry an odd number of passengers, so one rider has no seatmate — that's a free radical with a lone, unpaired electron. And big luxury cars from the third row of the periodic table (sulfur, phosphorus) have fold-down jump seats, so they can squeeze in extra passengers (more than eight electrons). Smaller second-row cars simply have no jump seats, so they can never be overfilled. (The analogy's "seats" are orbitals; the real reason is the availability of d orbitals in heavier atoms.)

Key takeaways

  • Three exception types: incomplete octet, odd-electron (radical), expanded octet.
  • Incomplete: BF₃ (B = 6 e⁻), BeCl₂ (Be = 4 e⁻), BCl₃.
  • Odd-electron: NO (11 e⁻), NO₂ (17 e⁻) — free radicals with an unpaired electron.
  • Expanded: SF₆ (S = 12 e⁻), PCl₅ (P = 10 e⁻), XeF₄ (Xe = 12 e⁻).
  • Expanded octets need a period-3 or heavier central atom (d orbitals available).
  • Second-period atoms (C, N, O, F, Ne) never exceed 8 valence electrons.
  • Electron-deficient species are Lewis acids.
  • Free radicals are highly reactive and often paramagnetic.
  • Exceptions: incomplete octet, odd-electron (radical), expanded octet.
  • Incomplete: BF₃ (6 e⁻), BeCl₂ (4 e⁻).
  • Odd-electron: NO (11 e⁻), NO₂ (17 e⁻) — one unpaired electron.
  • Expanded: SF₆ (12 e⁻), PCl₅ (10 e⁻), XeF₄.
  • Expanded octets require a period-3+ central atom (d orbitals).
  • Second-period atoms never exceed 8 electrons.
  • These species explain Lewis acidity, radical reactivity, and industrial gases.

Keep learning

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Practice General Chemistry I

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Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Identify the three categories of octet-rule exceptions.
  • Draw Lewis structures for electron-deficient, odd-electron, and expanded-octet species.
  • Explain why expanded octets occur only for period 3 and heavier central atoms.
  • Recognize these exceptions in real molecules (BF₃, NO, SF₆).

Sources & references

  1. OpenStax, *Chemistry 2e*, "7.3 Lewis Symbols and Structures."
  2. OpenStax, *Chemistry 2e*, "7.4 Formal Charges and Resonance."
  3. OpenStax, *Chemistry 2e*, "8.2 Hybrid Atomic Orbitals."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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