Exceptions to the Octet Rule
The octet rule is one of the most useful guidelines in chemistry - but it is not a law. Several important molecules flatly refuse to follow it, and the MCAT expects you to recognize all three categories of rule-breakers.
There are three categories of exceptions: incomplete octets, expanded octets, and odd-electron species. Let us look at each one.
Exception 1: Incomplete Octets
Some atoms are stable with fewer than 8 electrons in their valence shell. The most important examples for the MCAT are:
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Hydrogen (H): satisfied with 2 electrons (duet rule). Hydrogen’s valence shell is the 1s orbital, which holds a maximum of 2 electrons.
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Helium (He): also satisfied with 2 electrons, for the same reason.
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Beryllium (Be): stable with 4 electrons. BeCl₂ is a classic example - beryllium has only two bonds and no lone pairs, giving it just 4 valence electrons.
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Boron (B): stable with 6 electrons. BF₃ and BH₃ are the most commonly tested examples. In BF₃, boron forms three single bonds and has no lone pairs, giving it only 6 electrons around it.
Why do these atoms tolerate incomplete octets? They simply do not have enough valence electrons or enough orbitals to reach 8. Hydrogen has only one orbital in its valence shell (1s). Boron has only 3 valence electrons and tends to form only 3 bonds rather than forcing a fourth.
Exception 2: Expanded Octets
Elements in Period 3 and beyond can accommodate more than 8 electrons around them. This is called an expanded octet or hypervalency.
Lewis structures: count first, then draw
Scroll sideways to see the whole map.
Formal charge, in one lineFormal charge = valence electrons − lone-pair electrons − number of bonds. Work it out for every atom; the sum must equal the overall charge on the species. When two structures both obey the octet rule, the better one is the one with formal charges nearest zero, and with any negative charge sitting on the most electronegative atom.
Resonance is not flippingThe three carbonate structures are not states the ion moves between. The real ion is a single average of all three, which is why all three C–O bonds are identical in length — longer than a double bond, shorter than a single — and why the charge is spread evenly over all three oxygens.
Which elements may expandOnly period 3 and beyond. Nitrogen cannot have five bonds and oxygen cannot have three in a neutral structure, however tempting it looks; phosphorus and sulfur, one row down, can. That single restriction resolves most disputed Lewis structures on the exam.
The reason is simple: starting in Period 3, atoms have d orbitals available. These extra orbitals provide additional “seats” for electrons beyond the usual 8.
Common MCAT examples of expanded octets:
| Molecule | Central Atom | Electrons Around Central Atom | Geometry |
|----------|-------------|-------------------------------|----------|
| PCl₅ | Phosphorus | 10 | Trigonal bipyramidal |
| SF₆ | Sulfur | 12 | Octahedral |
| ClF₃ | Chlorine | 10 | T-shaped |
| XeF₂ | Xenon | 10 | Linear |
| IF₅ | Iodine | 12 | Square pyramidal |
| SO₄²⁻ | Sulfur | up to 12 | Tetrahedral |
Notice that phosphorus (Period 3), sulfur (Period 3), chlorine (Period 3), xenon (Period 5), and iodine (Period 5) are all in Period 3 or later. They all have accessible d orbitals.
How to Recognize an Expanded Octet
When you draw a Lewis structure and the central atom already has a full octet but there are still leftover electrons to place, put those extra electrons on the central atom as lone pairs - but only if the central atom is in Period 3 or beyond.
For example, in XeF₂: xenon has 8 valence electrons, each fluorine has 7, for a total of 22. After drawing two Xe-F bonds (4 electrons) and filling octets on fluorine (12 electrons for lone pairs), you have 6 electrons left. These go on xenon as three lone pairs, giving xenon 10 total electrons. This is acceptable because xenon is in Period 5.
Exception 3: Odd-Electron Species (Free Radicals)
If a molecule has an odd total number of valence electrons, it is mathematically impossible for every atom to have a full octet. At least one atom will be stuck with 7 electrons.
These molecules are called free radicals, and they are typically very reactive because that unpaired electron desperately seeks a partner.
Key MCAT examples:
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NO (nitric oxide): 5 + 6 = 11 valence electrons. Nitrogen has only 7 electrons around it. Despite being a radical, NO is an important biological signaling molecule (vasodilation, immune response).
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NO₂ (nitrogen dioxide): 5 + 2(6) = 17 valence electrons. Again, an odd number means at least one atom cannot have a full octet.
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O₂ (molecular oxygen): Although the Lewis structure appears to give every atom a full octet, molecular oxygen is actually a diradical with two unpaired electrons. This is a limitation of Lewis structures - they cannot always capture the full electronic picture. Molecular orbital theory handles this correctly.
Summary: Three Categories at a Glance
| Exception Type | What Happens | Key Examples | Why It Occurs |
|---------------|-------------|--------------|---------------|
| Incomplete octet | Fewer than 8 electrons | H (2), Be (4), B (6) | Not enough valence electrons or orbitals |
| Expanded octet | More than 8 electrons | PCl₅, SF₆, ClF₃, XeF₂ | d orbitals available (Period 3+) |
| Odd-electron species | Odd number of total electrons | NO, NO₂ | Cannot pair all electrons into full octets |
MCAT Strategy
When the MCAT gives you a molecule and asks about bonding, do a quick check:
- Is the central atom in Period 2? Then it must obey the octet rule (no exceptions for C, N, O, F).
- Is the central atom in Period 3 or later? Then an expanded octet is possible if needed.
- Is the total number of valence electrons odd? Then you have a free radical.
- Is the central atom boron or beryllium? Then an incomplete octet is expected.
Phosphorus is in Period 3 and has empty 3d orbitals that can accommodate extra electrons beyond 8. Nitrogen is in Period 2 and has no d orbitals available, so it can never exceed 8 electrons. This is why PCl₅ exists but NCl₅ does not.
1. Incomplete octets - atoms with fewer than 8 electrons (e.g., BF₃ - boron has only 6).
2. Expanded octets - atoms with more than 8 electrons (e.g., SF₆ - sulfur has 12).
3. Odd-electron species - molecules with an odd total electron count (e.g., NO - 11 total valence electrons, so at least one atom has only 7).
C, N, O, and F. All are in Period 2 and lack d orbitals. They can never accommodate more than 8 electrons in their valence shell. Remember: “C, N, O, F Never Overflow.”