Worked example 1
Explain why BF₃ is stable with only six electrons on boron, while NF₃ is not drawn that way.
Try it first: Count valence electrons for each molecule.
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Molecular StructureOctet ruleContent level: Challenge 20 min
Exceptions to the Octet Rule
What you'll be able to do: Recognise incomplete octets, odd-electron species and expanded octets, and explain when each is allowed.
Best after: Valence Electrons and the Octet Rule
Introduction
The octet rule works for most molecules you meet, but BF₃, NO and SF₆ all break it for very different reasons.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Identify electron-deficient central atoms
- Recognise odd-electron radicals from an odd valence total
- Explain why only period 3 and heavier atoms can expand their octet
- Decide whether a proposed structure is a legitimate exception
Lesson
Incomplete octets
Beryllium and boron form stable compounds with fewer than eight electrons around the central atom. BeCl₂ leaves beryllium with 4 electrons and BF₃ leaves boron with 6. These molecules are strong Lewis acids precisely because the centre is short of electrons, which is why BF₃ reacts eagerly with ammonia.
An incomplete octet is only acceptable for Be, B and occasionally Al. Never leave carbon, nitrogen or oxygen short.
Odd-electron species
If the total valence electron count is odd, no arrangement can pair every electron. Nitrogen monoxide NO has 11 valence electrons and nitrogen dioxide NO₂ has 17. One unpaired electron is left on the least electronegative atom, and the species is a radical, which is why these molecules are so reactive.
Expanded octets
Central atoms in period 3 and below, such as P, S, Cl and Xe, can hold 10 or 12 electrons in structures like PCl₅ and SF₆. The traditional explanation is that empty 3d orbitals are energetically accessible. Period 2 atoms have only 2s and 2p orbitals, giving four orbitals and a hard ceiling of eight electrons.
Never expand the octet of C, N, O or F. This single rule eliminates most wrong Lewis structures on tests.
Deciding which exception applies
Count the total valence electrons first. An odd total means a radical. An even total with too few electrons to complete the centre points to an electron-deficient atom. An even total with electrons left over after every octet is filled means the extras go on a period 3 or heavier central atom.
Use electron parity to spot a radical
An odd total number of valence electrons means at least one electron remains unpaired in any Lewis structure. Such a radical cannot give every atom a conventional pair-only octet. This is a different exception from an electron-deficient central atom such as B or an expanded-valence central atom from period 3 or below. Classify the reason for the exception before repairing a drawing.
Odd electron count signals a radical; an even count does not guarantee full octets.
Key ideas
Rule
Period 2 ceiling
C, N, O and F can never hold more than eight valence electrons.
Definition
Radical
A species with an odd number of valence electrons and therefore one unpaired electron.
Key concept
Electron deficient
BF₃ leaves boron with only six electrons and readily accepts a lone pair.
Key concept
Expanded
SF₆ places twelve electrons around sulfur in six bonding pairs.
Worked examples
Worked example 2
Can the central atom in ClF₃ hold more than eight electrons? Support your answer.
Try it first: Check the period of the central atom.
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Worked example 3
Why must neutral NO be an odd-electron exception?
Try it first: Count N and O valence electrons.
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Common mistakes
Expanding the octet of nitrogen or oxygen to remove a formal charge.
Why it's wrong: Period 2 atoms have only four valence orbitals and cannot exceed eight electrons.
Check instead: Keep period 2 centres at eight and accept the formal charges.
Treating any odd count as a mistake in arithmetic.
Why it's wrong: Radicals such as NO genuinely have an odd number of valence electrons.
Check instead: Recount once, and if the total is still odd, draw a radical.
Assuming every period 3 central atom must expand.
Why it's wrong: Expansion happens only when there are surplus electrons or a clear formal-charge benefit.
Check instead: Fill all octets first, then place leftovers on the central atom.
After the lesson · Question bank
Independent practice
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What you should now know
Three families of species break the octet rule: electron-deficient centres such as Be and B, odd-electron radicals such as NO and NO₂, and expanded octets on period 3 and heavier central atoms that have accessible d orbitals. Period 2 elements can never expand beyond eight because their valence shell has only four orbitals.
- Be and B commonly form stable incomplete octets
- An odd valence total means a radical with one unpaired electron
- Period 3 and heavier central atoms can expand to 10 or 12 electrons
- Period 2 atoms are capped at eight electrons with no exceptions
- Count the total first, then decide which exception applies
Sources and further reading
- Chemistry 2e, Section 7.3: Lewis Symbols and StructuresOpenStax · Rice University · 7.3View source
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