VSEPR Theory & Molecular Shapes VSEPR: the simple rule that gives 3D shape Why shape matters: A molecule’s 3D shape decides polarity, boiling point, reactivity, enzyme binding, and catalyst selectivity. Valence Shell Electron Pair Repulsion (VSEPR) theory says electron pairs (bonding and lone) around a central atom spread out to minimize repulsion. From just counting electron domains, you can predict geometry and bond angles for most main-group molecules on NEET. Electron domains arrange to stay far apart: SN 2 → linear, SN 3 → trigonal planar, SN 4 → tetrahedral, SN 5 → trigonal bipyramidal, SN 6 → octahedral. Electron domain (electron group) A region of electron density around the central atom: a single bond, double bond, triple bond, or a lone pair each counts as one domain. An electron domain that is a bonding pair shared with a surrounding atom. Bond pair (BP) An electron domain consisting of a nonbonding pair on the central atom; occupies more space than a bond pair and compresses bond angles. Lone pair (LP) Total electron domains around the central atom: SN = bond pairs + lone pairs. Steric number (SN) Arrangement of all electron domains (BPs + LPs) around the central atom. Electron geometry Molecular shape (molecular geometry) Arrangement of only the atoms (ignore lone pairs when naming the shape). AXmEn notation A = central atom, X = surrounding atoms (m), E = lone pairs on central (n). Example: H2O is AX2E2. Steric number definition Add bonding pairs and lone pairs on the central atom to get SN. Shortcut for SN V = valence electrons of central atom; M = number of monovalent atoms attached; C = cationic charge; A = anionic charge. Ideal electron geometries by SN (2–4) Ideal electron geometries by SN (5–6) Lone pairs repel more strongly and compress adjacent bond angles. Repulsion order Multiple bonds count as ONE electron domain for SN. But a double or triple bond is slightly bulkier than a single bond and can cause small angle tweaks around it. remember Predicting shape: the 3-step AXE method Count the steric number (SN) for the central atom (use either direct counting or the shortcut). Assign electron geometry from SN (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral). Place lone pairs to minimize repulsion (equatorial preferred in trigonal bipyramidal; in octahedral, positions are equivalent). Ignore LPs when you name the molecular shape; keep their bond-angle effects. Steps Electron geometry by steric number Cases SN Electron Geometry Ideal Bond Angles Simple Example AXE Linear 180° CO2 (carbon dioxide) O=C=O; SMILES: O=C=O AX2 Trigonal planar 120° BF3 (boron trifluoride) B(F)(F)F AX3 Tetrahedral 109.5° CH4 (methane) C AX4 Trigonal bipyramidal 90°, 120° PF5 (phosphorus pentafluoride) F[P](F)(F)(F)F AX5 Octahedral 90° SF6 (sulfur hexafluoride) F[S](F)(F)(F)(F)F AX6 Lone pairs change shapes and angles Lone pairs occupy more space than bond pairs. So they push bonds closer together, reducing bond angles. In trigonal bipyramidal (TBP), lone pairs prefer equatorial positions (120° around, fewer 90° interactions). In octahedral, all positions are equivalent, but two lone pairs prefer trans (opposite) to minimize LP–LP repulsion. Learn the classic distortions: SN 4 with 1 LP: NH3 (ammonia, trigonal pyramidal, ~107°); SMILES: N (NH3 is neutral but trigonal pyramidal around N). SN 4 with 2 LP: H2O (water, bent/V-shaped, ~104.5°); SMILES: O SN 5 with 1 LP: SF4 (see-saw; LP equatorial); SMILES: F[S](F)(F)F SN 5 with 2 LP: ClF3 (T-shaped; both LP equatorial); SMILES: FCl(F)F SN 5 with 3 LP: XeF2 (linear; all 3 LP equatorial); SMILES: F[Xe]F SN 6 with 1 LP: BrF5 (square pyramidal); SMILES: FBr(F)(F)(F)F SN 6 with 2 LP: XeF4 (square planar; LPs trans); SMILES: F[Xe](F)(F)F Common VSEPR shapes with angles: linear CO2 (180°), trigonal planar BF3 (120°), tetrahedral CH4 (109.5°), trigonal pyramidal NH3 (~107°), bent H2O (~104.5°), trigonal bipyramidal PCl5 (90°, 120°), octahedral SF6 (90°). Lone pairs shown as larger lobes. Angle compression by lone pairs: CH4 (109.5°) > NH3 (~107°) > H2O (~104.5°). Lone pairs repel more, so they squeeze bond angles. TBP placement rule: Put lone pairs in equatorial positions first. That’s why SF4 is see-saw (LP equatorial), ClF3 is T-shaped (two equatorial LPs), and XeF2 is linear (three equatorial LPs). neet-alert Linear Linear 180° CO2 (O=C=O) AX2 Trigonal planar Trigonal planar 120° BF3 (B(F)(F)F) AX3 Trigonal planar Bent ~120° SO2 (O=S=O with lone pair) [not needed for NEET angles] AX2E Tetrahedral Tetrahedral 109.5° CH4 (C) AX4 Tetrahedral Trigonal pyramidal ~107° NH3 (N) AX3E Tetrahedral Bent ~104.5° H2O (O) AX2E2 Trigonal bipyramidal Trigonal bipyramidal 90°, 120° PF5 (F[P](F)(F)(F)F) AX5 Trigonal bipyramidal See-saw <90°, <120° SF4 (F[S](F)(F)F) AX4E Trigonal bipyramidal T-shaped ~90° ClF3 (FCl(F)F) AX3E2 Trigonal bipyramidal Linear 180° XeF2 (F[Xe]F) AX2E3 Octahedral Octahedral 90° SF6 (F[S](F)(F)(F)(F)F) AX6 Octahedral Square pyramidal ≈90° BrF5 (FBr(F)(F)(F)F) AX5E Octahedral Square planar 90° XeF4 (F[Xe](F)(F)F) AX4E2 Representative molecular shapes by SN, BP, LP (high-yield set) Case SN BP LP Electron Geometry Molecular Shape Approx. Angles Example (SMILES) AXE CH4 (methane) Tetrahedral 109.5° NH3 (ammonia) Trigonal pyramidal ~107° H2O (water) Bent ~104.5° Molecule SN LP on central Molecular shape Bond angle Examples Bond-angle compression by lone pairs (classic NEET trio) remember Water’s bent shape (AX2E2) makes the molecule polar and able to hydrogen-bond strongly. That’s why water is an exceptional solvent and crucial for biological transport and temperature regulation. Octahedral derivatives and related high-coordinate fluorides Starting from octahedral electron geometry (SN 6), adding lone pairs changes shape in a predictable way: AX6 (octahedral) → AX5E (square pyramidal) → AX4E2 (square planar). Some related halogen fluorides with fewer domains (SN 5) show TBP-based shapes (T-shaped, linear). The table shows both series clearly labeled by SN to avoid confusion. Compound SN Electron geometry AXE Observed molecular shape Series From octahedral and TBP to observed shapes in fluorides SF6 Octahedral AX6 Octahedral IF5 Octahedral AX5E Square pyramidal XeF4 Octahedral AX4E2 Square planar (LPs trans) BrF3 Trigonal bipyramidal AX3E2 T-shaped (LPs equatorial) XeF2 Trigonal bipyramidal AX2E3 Linear (3 LPs equatorial) tip For octahedral derivatives, place 2 LPs opposite (trans) to get square planar (e.g., XeF4). For TBP derivatives, always fill equatorial with LPs first. Industrial and biological relevance of shape Catalyst selectivity: Square planar Pt(II) complexes approach and activate H2 and alkenes differently from tetrahedral catalysts, tuning hydrogenation pathways. Polymerization: Tetrahedral metallocene catalysts create defined tacticity in olefin polymerization by precise monomer approach angles. Biology: Shape complementarity underlies drug–receptor binding; V-shaped H2O is polar, enabling hydrogen bonding networks vital for life. Common NEET traps and fixes Molecular geometry equals electron domain geometry. Electron geometry includes all electron domains (BPs + LPs). Molecular geometry includes only the positions of atoms; lone pairs are ignored in the name but still affect angles. Any multiple bond counts as a single electron domain for SN (though it repels a bit more than a single bond). Miscounting inflates SN and gives the wrong shape. Double/triple bonds count as multiple domains in steric number. Lone pairs don’t change much; LPs and BPs are equivalent. LPs occupy more space and compress bond angles more strongly: LP–LP > LP–BP > BP–BP. Never treat LP as equivalent to BP for angle predictions. neet-alert “Shape vs geometry” questions are common. Example: For NH3, electron geometry is tetrahedral (SN 4), molecular shape is trigonal pyramidal (AX3E), H–N–H angle ~107°. Worked mini-examples with AXE CO2: Central C, SN = 2 (2 BPs, 0 LP) → linear electron geometry; shape = linear; AX2; angle = 180°; SMILES: O=C=O. BCl3: Central B, SN = 3 (3 BPs, 0 LP) → trigonal planar; AX3; 120°; SMILES: B(Cl)(Cl)(Cl). CH4: Central C, SN = 4 (4 BPs, 0 LP) → tetrahedral; AX4; 109.5°; SMILES: C. NH3: Central N, SN = 4 (3 BPs, 1 LP) → tetrahedral electron geometry; shape trigonal pyramidal; AX3E; ~107°; SMILES: N. H2O: Central O, SN = 4 (2 BPs, 2 LP) → tetrahedral electron geometry; shape bent; AX2E2; ~104.5°; SMILES: O. PCl5: Central P, SN = 5 (5 BPs, 0 LP) → trigonal bipyramidal; AX5; 90°, 120°; SMILES: ClP(Cl)(Cl)(Cl)Cl. SF4: Central S, SN = 5 (4 BPs, 1 LP) → see-saw (LP equatorial); AX4E; angles <90°, <120°; SMILES: F[S](F)(F)F. ClF3: Central Cl, SN = 5 (3 BPs, 2 LP) → T-shaped; AX3E2; ~90°; SMILES: FCl(F)F. XeF2: Central Xe, SN = 5 (2 BPs, 3 LP) → linear; AX2E3; 180°; SMILES: F[Xe]F. SF6: Central S, SN = 6 (6 BPs, 0 LP) → octahedral; AX6; 90°; SMILES: F[S](F)(F)(F)(F)F. BrF5: Central Br, SN = 6 (5 BPs, 1 LP) → square pyramidal; AX5E; ≈90°; SMILES: FBr(F)(F)(F)F. XeF4: Central Xe, SN = 6 (4 BPs, 2 LP) → square planar; AX4E2; 90°; SMILES: F[Xe](F)(F)F. tip Fast AXE memory: SN 2 L; SN 3 TP; SN 4 Td; SN 5 TBP; SN 6 Oh. With LPs: Td → 1 LP pyramidal, 2 LP bent. TBP → 1 LP see-saw, 2 LP T, 3 LP linear. Oh → 1 LP square pyramidal, 2 LP square planar. Glossary Electron pairs around a central atom repel and arrange to minimize repulsions, setting geometry and angles. VSEPR theory Number of electron domains = bond pairs + lone pairs. Steric number (SN) A bonding electron pair shared with a surrounding atom. Bond pair (BP) A nonbonding pair on the central atom; repels more than a bond pair. Lone pair (LP) Layout of all electron domains (BPs + LPs) around the central atom. Electron geometry Layout of only atoms around the central atom (ignore LPs in the name). Molecular shape A = central atom, X = number of bonded atoms m, E = number of lone pairs n. AXmEn notation SN 5 electron geometry with 3 equatorial and 2 axial positions (90° and 120°). Trigonal bipyramidal (TBP) TBP-derived molecular shape with 1 equatorial lone pair (AX4E). See-saw TBP-derived molecular shape with 2 equatorial lone pairs (AX3E2). T-shaped Octahedral-derived molecular shape with 1 lone pair (AX5E). Square pyramidal Octahedral-derived molecular shape with 2 trans lone pairs (AX4E2). Square planar Limits: VSEPR works best for main-group molecules. Transition-metal complex shapes often follow Crystal Field Theory (CFT) and ligand-field effects (covered in Coordination Compounds). neet-alert