Exceptions to the Octet Rule: Incomplete, Expanded and Odd-Electron Molecules

NCERT lists exactly three. (1) Incomplete octet: the central atom has fewer than 8 electrons, like LiCl, BeH2, BeCl2, BCl3, BF3, AlCl3. This happens with atoms that have less than 4 valence electrons (Li has 1, Be has 2, B has 3). (2) Expanded octet: the central atom has more than 8 electrons, like PCl5 (10), SF6 (12), because these atoms can use empty d-orbitals. (3) Odd-electron molecules: the total number of electrons is odd, so at least one atom cannot reach 8, like NO and NO2.

An incomplete octet means the central atom has fewer than 8 electrons around it. Common NEET examples: BeCl2 and BeH2 (Be has only 4 electrons), BF3, BCl3 and AlCl3 (B and Al have only 6 electrons). These are also called electron-deficient molecules because they are 'hungry' for more electrons, so they act as Lewis acids (they accept electron pairs). This is why BF3 easily forms BF3.NH3 by taking a lone pair from nitrogen.

In PCl5 the central phosphorus atom forms 5 bonds, so it is surrounded by 5 bond pairs = 10 electrons, which is more than 8. This is an expanded octet. It is possible because phosphorus is in Period 3, so it has empty 3d-orbitals it can use for bonding. Second-period elements like C, N, O, F cannot do this because they have no d-orbitals in their valence shell. SF6 (12 electrons) is the classic expanded-octet example alongside PCl5.

Boron is in Period 2. Its valence shell is n=2, which has only 2s and 2p orbitals and NO d-orbitals. So boron can hold at most 8 electrons (4 bonds) and cannot go beyond covalency 4. Phosphorus and sulphur are in Period 3, where empty 3d-orbitals become available, so they can form more than 4 bonds. This is exactly why boron cannot form BF6 3- ion but aluminium (Period 3) can form AlF6 3-.

Yes. Count the valence electrons: N has 5 and each O has 6, so NO2 has 5 + 6 + 6 = 17 electrons, which is odd. With an odd total, it is impossible for every atom to get a perfect octet of 8, so one electron stays unpaired. NO (nitric oxide) is the other standard example: N(5) + O(6) = 11, also odd. Odd-electron molecules are usually paramagnetic and reactive. NCERT names both NO and NO2 as odd-electron molecules.

They mean the same thing here. 'Incomplete octet' describes the structure — the central atom has fewer than 8 electrons. 'Electron-deficient' describes the behaviour — because it lacks electrons, the molecule accepts electron pairs and acts as a Lewis acid. So BF3 has an incomplete octet AND is electron-deficient. NEET often uses the phrase 'electron deficient' in the question and expects you to recall the incomplete octet (6 electrons around B).

Incomplete octet (central atom has fewer than 8 electrons, e.g. BeCl2, BF3), expanded octet (central atom has more than 8 electrons, e.g. PCl5, SF6) and odd-electron molecules (odd total electrons so a perfect octet is impossible, e.g. NO, NO2).

No. Sulphur in SF6 makes 6 bonds and is surrounded by 12 electrons, far more than 8. This is an expanded octet, made possible by sulphur's empty 3d-orbitals. SF6 is a favourite NEET example of an expanded octet.

Yes. Because they have an unpaired electron (odd total), molecules like NO and NO2 are paramagnetic, meaning they are attracted by a magnetic field. They are also generally reactive.

In BF3 boron has only 6 electrons (incomplete octet), so it is electron-deficient and acts as a Lewis acid. In NF3, nitrogen has 3 bond pairs plus 1 lone pair = 8 electrons, a complete octet, so it is not electron-deficient.

Elements of Period 3 and beyond (P, S, Cl, Xe, etc.) can show expanded octets because their valence shell has empty d-orbitals. Period 2 elements (C, N, O, F) cannot, so they never exceed 8 electrons.