Read Hybridisation From Electron Domains, Not From Memory
Hybridisation follows directly from electron-domain count — 2 domains is sp, 3 is sp2, 4 is sp3, 5 is sp3d, 6 is sp3d2 — so any hybridisation question is really an electron-domain-counting question in disguise.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Hybridisation changes when a lone pair becomes a bonding pair, or vice versa
BF3 (sp2, trigonal planar, 3 domains) becomes sp3 tetrahedral BF4- when it accepts a fourth bonding pair from F-; NH3 (sp3, 4 domains including the lone pair) stays sp3 as NH4+ since the lone pair simply becomes a fourth bonding pair without changing the domain count — the hybridisation-change question always reduces to 'did the total domain count actually change?'
Valence limits: why some formulas can't exist
Second-period elements (N, O, F) cannot exceed 4 electron domains because they have no accessible d-orbitals to expand into — this is why NCl5 and OF4 don't exist even though PCl5 and SF4 do. Third-period-and-below elements CAN expand their octet using d-orbitals (or, in modern treatments, through more delocalised bonding), which is the real dividing line for 'which hypervalent molecule can exist.'
Percentage s-character and Bent's rule
When one central atom has multiple non-equivalent bonds (like P in P4, or a phosphorus centre with different substituents), electronegative substituents pull more p-character toward themselves, leaving more s-character concentrated in bonds to less electronegative groups (or to lone pairs) — this is Bent's rule, and it explains why not every bond from a formally 'sp3' centre is geometrically identical.
OlympIQ exam lens
- ✓Electron domain count maps directly to hybridisation: 2→sp, 3→sp2, 4→sp3, 5→sp3d, 6→sp3d2.
- ✓A lone pair converting to a bonding pair (or vice versa) changes hybridisation only if the total domain count changes.
- ✓Second-period elements (N, O, F) cap at 4 domains — no d-orbitals to expand into; period 3+ elements can exceed 4.
- ✓Bent's rule: electronegative substituents pull p-character toward themselves, concentrating s-character elsewhere on the same central atom.
Don't assume adding a fourth group to a 3-domain sp2 centre (like BF3→BF4-) automatically means 'no hybridisation change' — check the domain count in each species independently rather than assuming the formula alone tells you.
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