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.
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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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.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
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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