On this page
Questions that ask "which of these does not obey the octet rule?" look like memory tests, but they are really counting problems. If you can count valence electrons and place them in a Lewis structure, you can sort any molecule into the right group. This post gives NCERT's three exceptions, a three-step check, seven worked molecules and the slips to avoid.
What does NCERT say the octet rule cannot explain?
Section 4.1.5 of the NCERT Class 11 chapter Chemical Bonding and Molecular Structure says the octet rule "is not universal" and "applies mainly to the second period elements". It lists three types of exception:
Incomplete octet of the central atom: fewer than eight electrons, mainly for elements with fewer than four valence electrons. Examples: LiCl, BeH₂, BCl₃, AlCl₃ and BF₃.
Odd-electron molecules: an odd total, as in NO and NO₂, so not every atom can have eight.
Expanded octet: more than eight electrons around the central atom, for elements in the third period and beyond. Examples: PF₅, SF₆, H₂SO₄ and many coordination compounds.
NCERT adds three more drawbacks. Noble gases such as xenon and krypton form compounds like XeF₂, KrF₂ and XeOF₂. The rule says nothing about the shape of molecules. It also says nothing about their energy or relative stability. The JEE (Main) 2026 syllabus lists the Kössel-Lewis approach and VSEPR shapes, so expect these molecules in both board and entrance papers.
A three-step check for any molecule
Add up the valence electrons. Use group numbers, then subtract one for each positive charge and add one for each negative charge. If the total is odd, it is an odd-electron species. You can stop.
Draw the skeleton and place the electrons. Single bonds first, then lone pairs on the outer atoms, then any leftovers on the central atom.
Count around the central atom. Count each bond as two and each lone pair as two. Fewer than eight is an incomplete octet. More than eight is an expanded octet, and it is only possible for elements from period 3 onwards.
Worked examples
BF₃. Valence electrons: 3 + 3 × 7 = 24. Three B–F bonds use 6, and three lone pairs on each F use 18. Total 24. Boron has only 6 around it: incomplete octet.
NO₂. 5 + 2 × 6 = 17. Odd, so it is an odd-electron molecule. In NCERT's structure the nitrogen holds the unpaired electron: one double bond (4), one single bond (2) and one lone electron make 7.
NO₂⁺. 5 + 12 − 1 = 16. Even, so it is not an odd-electron species. One charge changes the answer.
PCl₅. 5 + 5 × 7 = 40. Five bonds put 10 electrons on P, and each Cl carries three lone pairs (30). Total 40. Phosphorus has 10: expanded octet.
SF₆. 6 + 6 × 7 = 48. Six bonds give 12 on S, and the fluorines' lone pairs use 36. Expanded octet with 12.
SF₄. 6 + 4 × 7 = 34. Four bonds (8) plus the fluorines' lone pairs (24) use 32, so the last 2 go on sulphur as a lone pair. Sulphur has 8 + 2 = 10. Four bonds can still mean an expanded octet.
SCl₂. 6 + 2 × 7 = 20. Two bonds (4) plus chlorine lone pairs (12) leave 4, which go on S as two lone pairs. Sulphur has exactly 8. NCERT uses this to show that sulphur also obeys the rule in many compounds.
Why can only period 3 elements and beyond expand the octet?
NCERT's explanation is that these elements have 3d orbitals available for bonding, alongside their 3s and 3p orbitals. A second-period atom such as nitrogen has no such orbitals in its valence shell, so it cannot hold more than eight. That is why an exam can pair PCl₅, which exists, with NCl₅, which does not. For school and JEE answers, give the NCERT reason. The JEE (Advanced) 2026 syllabus also asks for "hybridisation involving s, p and d orbitals" and for trigonal bipyramidal and octahedral shapes, which are exactly the shapes of PCl₅ and SF₆.
Once you have the electron count, the shape follows from VSEPR. Our post on predicting molecular shapes with VSEPR takes SF₄ and XeF₂ from count to geometry. XeF₂ (8 + 14 = 22 electrons) leaves three lone pairs on xenon, giving it 10 electrons in all.
Common mistakes and how to avoid them
Calling any electron-poor molecule "odd-electron". BF₃ is short of eight but has an even total. Odd-electron means the whole molecule has an odd count.
Forgetting the charge. NO₂ and NO₂⁺ differ by one electron and fall in different groups.
Counting lone pairs only on the outer atoms. Leftover electrons go on the central atom, as SF₄ and XeF₂ show.
Expanding the octet of N, O or C. Second-period atoms stop at eight.
Assuming one structure fits every resonance case. When two structures both satisfy the counts, formal charge helps you choose. Our guide to formal charge and Lewis structures covers that step.
A practice routine
Make a list of 15 species: the seven above plus BeCl₂, AlCl₃, ClO₂, PF₅, H₂SO₄, NO, XeF₄ and SO₄²⁻.
For each, write the valence total and the count around the central atom before naming the type.
Check yourself against NCERT section 4.1.5.
Two days later, mix in five shape questions so that count, exception and geometry come together.
If a molecule keeps tripping you up because the idea of valence electrons feels shaky, Fermi's Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can go back to the step you skipped.
Start with SF₄, ClO₂ and BeCl₂ today: count first, then classify.
