Chemistry · Chemical Bonding · NEET
An expanded octet is when the central atom of a molecule has MORE than 8 electrons in its valence (outer) shell. Normally atoms want exactly 8 electrons (the octet rule). But some atoms break this rule and hold 10, 12, or even 14 electrons. Example: in PF5, phosphorus makes 5 bonds, so it has 5 x 2 = 10 electrons around it. In SF6, sulphur makes 6 bonds, so it has 12 electrons. This is called an expanded octet, and such molecules are called hypervalent.
This is the most asked doubt. Both N and P are in Group 15, so both have 5 valence electrons. But nitrogen is in period 2 — it only has 2s and 2p orbitals, which together hold a MAXIMUM of 8 electrons. Nitrogen has NO d orbitals, so it can never go beyond an octet. Phosphorus is in period 3 — it has 3s, 3p AND empty 3d orbitals. Phosphorus can use these 3d orbitals to make room for extra electrons, so it forms PF5 with 10 electrons. Rule to remember: no d orbitals means no expanded octet.
NCERT says it clearly: elements in and beyond the third period have 3d orbitals available for bonding, in addition to 3s and 3p. The energy of the 3d orbitals is close enough to 3s and 3p to be used for bonding. Period 2 elements (C, N, O, F) do not have this — their outer shell (n=2) has only s and p orbitals, capped at 8 electrons. So expanded octet is only seen in P, S, Cl, Br, I, Xe and similar heavier atoms.
Count the bonds and multiply by 2 (each bond = 1 shared pair = 2 electrons). In PF5, phosphorus forms 5 single bonds to 5 fluorine atoms, so 5 x 2 = 10 electrons around P. In SF6, sulphur forms 6 single bonds to 6 fluorine atoms, so 6 x 2 = 12 electrons around S. Both are more than 8, so both have an expanded octet.
Yes. NCERT lists three exceptions (limitations) of the octet rule: (1) incomplete octet, like BCl3 with only 6 electrons; (2) odd-electron molecules, like NO and NO2; and (3) expanded octet, like PF5, SF6, PCl5, SF4, XeF2. Expanded octet is the third type of exception, where the atom has MORE than 8 electrons instead of exactly 8.
Hypervalent means the central atom has a valence (number of bonds) that is too high for a simple octet. In other words, a hypervalent molecule is one whose central atom carries an expanded octet — more than 8 electrons. PF5 (10 electrons), SF6 (12 electrons), and IF7 (14 electrons) are all hypervalent. The hybridisation matches: PF5 is sp3d, SF6 is sp3d2, IF7 is sp3d3 — the d orbitals are how the extra space is made.
No, this is a common trap. NCERT points out that sulphur also forms compounds that OBEY the octet rule. For example, in sulphur dichloride (SCl2), the sulphur atom has a normal octet of 8 electrons. Having d orbitals available does NOT force expansion — it just makes expansion POSSIBLE. So SF6 expands (12 electrons) but SCl2 does not. Look at the actual number of bonds each time.
Amongst the following, the total number of species NOT having eight electrons around the central atom in its outermost shell is: NH3, AlCl3, BeCl2, CCl4, PCl5
The correct statement is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
PF5 (10 electrons, sp3d), SF6 (12 electrons, sp3d2), PCl5 (10 electrons, sp3d), SF4 (10 electrons), XeF2 and XeF4, and IF7 (14 electrons, sp3d3). All have a central atom from period 3 or below with more than 8 valence electrons.
Sulphur is in period 3, so it has empty 3d orbitals whose energy is close to 3s and 3p. It uses sp3d2 hybridisation (mixing one 3s, three 3p and two 3d orbitals) to form six equal S-F bonds, giving an octahedral shape with 12 electrons around sulphur.
Yes. The octet rule is a useful guide, mainly for period-2 elements, but it is not universal. Expanded octet is one of the three official NCERT exceptions to the octet rule, along with incomplete octet and odd-electron molecules.
Any hybridisation that uses d orbitals: sp3d (5 electron pairs, like PF5, PCl5), sp3d2 (6 pairs, like SF6), or sp3d3 (7 pairs, like IF7). Using d orbitals is what allows more than four bonds, and hence more than 8 electrons.
Yes, very. NEET regularly asks to identify exceptions to the octet rule (like the 2023 count question with PCl5) and to link period, d orbitals and maximum covalency (like the 2026 Boron question). Knowing PF5 = 10 and SF6 = 12 electrons, and that only period-3+ atoms can expand, wins these marks quickly.
