Inorganic · Chemical Bonding
Chemical Bonding — VSEPR, Hybridization & MO Theory
Exam-focused revision notes on Chemical Bonding — VSEPR, Hybridization & MO Theory for CSIR-NET Chemical Sciences, GATE Chemistry and IIT-JAM — the core concepts, the formulas worth memorising, the traps that cost marks, and a quick-recall table.
VSEPR: predicting shape
- Electron pairs (bonding + lone pairs) around the central atom arrange to minimise repulsion.
- Repulsion order: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.
- Electron-pair geometry counts all pairs; molecular shape counts only the atoms (lone pairs are "invisible" but still push).
- AXmEn notation: A = central atom, X = bonded atoms (m), E = lone pairs (n).
Steric number = (bonded atoms) + (lone pairs on central atom) — this sets the electron-pair geometry & hybridization.
The five parent geometries
- 2 → linear (180°), e.g. BeCl2, CO2.
- 3 → trigonal planar (120°), e.g. BF3; with 1 lone pair → bent (SO2).
- 4 → tetrahedral (109.5°), e.g. CH4; 1 LP → trigonal pyramidal (NH3, ~107°); 2 LP → bent (H2O, ~104.5°).
- 5 → trigonal bipyramidal, e.g. PCl5; lone pairs go equatorial first → see-saw (SF4), T-shape (ClF3), linear (XeF2).
- 6 → octahedral (90°), e.g. SF6; 1 LP → square pyramidal (BrF5); 2 LP (trans) → square planar (XeF4).
Hybridization → shape
- sp (SN 2) → linear.
- sp² (SN 3) → trigonal planar.
- sp³ (SN 4) → tetrahedral.
- sp³d (SN 5) → trigonal bipyramidal.
- sp³d² (SN 6) → octahedral.
Note the steric number ties VSEPR and hybridization together — count electron domains, and both fall out.
MO theory — homonuclear diatomics
- Atomic orbitals combine into bonding (lower energy) and antibonding (*, higher energy) molecular orbitals.
- Fill by the Aufbau/Hund rules, just like atoms.
Bond order = ½ (bonding electrons − antibonding electrons)
- s–p mixing raises σ2pz above the π2p pair for the lighter elements (B2, C2, N2) — this is the "swap".
- Ordering for B2, C2, N2: σ2s < σ*2s < π2px=π2py < σ2pz < π*2p < σ*2p.
- Ordering for O2, F2 (no swap): σ2s < σ*2s < σ2pz < π2px=π2py < π*2p < σ*2p.
- O2 is paramagnetic — two unpaired electrons in the degenerate π*2p orbitals (bond order 2). MO theory predicts this; Lewis structures don't.
- B2 is also paramagnetic (2 unpaired e− in π2p), a direct consequence of the s–p mixing swap.
- Bond orders: N2 = 3 (very strong, diamagnetic), O2 = 2, F2 = 1.
Bond order ↔ length ↔ energy
- Higher bond order → shorter bond → higher bond energy.
- Example trend: N2 (BO 3, shortest, strongest) > O2 (BO 2) > F2 (BO 1, longest, weakest).
- Removing an antibonding electron raises bond order (e.g. O2+ has BO 2.5, stronger than O2).
⚠️ Common traps students miss
- Electron geometry ≠ molecular shape. H2O is tetrahedral electron geometry but bent shape — lone pairs count for geometry, not for the reported shape.
- The MO ordering swaps at N2/O2: σ2pz is below π2p for B2–N2, but above π2p for O2, F2.
- O2 is paramagnetic — a classic MCQ. Don't call it diamagnetic.
- In trigonal bipyramidal, lone pairs occupy equatorial positions (less repulsion), not axial.
- Bond order can be fractional for ions — always recount bonding vs antibonding electrons.
30-second recall table
| Steric no. | Hybridization | Electron-pair geometry / shape (no LP) |
|---|---|---|
| 2 | sp | Linear |
| 3 | sp² | Trigonal planar |
| 4 | sp³ | Tetrahedral |
| 5 | sp³d | Trigonal bipyramidal |
| 6 | sp³d² | Octahedral |
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