Valence electrons on Cl: 7 (Group 17)
Steric number = ½(valence electrons + monovalent atoms attached) = ½(7 + 3) = 5
Electron pairs around Cl: 5 total
Bond pairs = 3 (three Cl−F bonds)
Lone pairs = 5 − 3 = 2 lone pairs
VSEPR geometry: 5 electron pairs = trigonal bipyramidal arrangement.
2 lone pairs occupy equatorial positions (more space) → 3 bond pairs: 2 equatorial + 1 axial... wait — lone pairs prefer equatorial in trigonal bipyramidal.
Actual arrangement: 2 lone pairs in equatorial positions, 3 F atoms: 2 axial + 1 equatorial → gives T-shape
ClF₃: Steric number = 5
3 bond pairs + 2 lone pairs
Geometry: T-shaped (from trigonal bipyramidal)
Lone pairs on Cl = 2
Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular geometry based on electron pair repulsions. Electron pairs (bonding and lone pairs) arrange themselves to minimize repulsion. Repulsion order: Lone pair–Lone pair (lp-lp) > Lone pair–Bond pair (lp-bp) > Bond pair–Bond pair (bp-bp). Lone pairs occupy more angular space than bond pairs (they are closer to central atom, more diffuse). Multiple bonds count as single region of electron density. Steric number = (number of atoms bonded to central atom) + (number of lone pairs on central atom).
| Steric No. | BP | LP | Shape | Example |
|---|---|---|---|---|
| 2 | 2 | 0 | Linear | BeCl₂, CO₂ |
| 3 | 3 | 0 | Trigonal planar | BF₃, SO₃ |
| 3 | 2 | 1 | Bent/V-shape | SO₂, SnCl₂ |
| 4 | 4 | 0 | Tetrahedral | CH₄, NH₄⁺ |
| 4 | 3 | 1 | Trigonal pyramidal | NH₃, PCl₃ |
| 4 | 2 | 2 | Bent | H₂O, H₂S |
| 5 | 5 | 0 | Trigonal bipyramidal | PCl₅ |
| 5 | 4 | 1 | See-saw | SF₄, IF₄⁺ |
| 5 | 3 | 2 | T-shaped | ClF₃, BrF₃ |
| 5 | 2 | 3 | Linear | XeF₂, I₃⁻ |
| 6 | 6 | 0 | Octahedral | SF₆, [Co(NH₃)₆]³⁺ |
| 6 | 5 | 1 | Square pyramidal | IF₅, BrF₅ |
| 6 | 4 | 2 | Square planar | XeF₄, [Pt(NH₃)₂Cl₂] |
ClF₃ has steric number 5 (trigonal bipyramidal parent). In trigonal bipyramidal geometry, two positions are axial (90° to equatorial) and three are equatorial (120° to each other). Lone pairs prefer equatorial positions because equatorial positions have less repulsion — they only have 2 bonds at 90° (the axial bonds), while axial positions have 3 bonds at 90° (the equatorial bonds). With 2 lone pairs: both go to equatorial positions, leaving 3 F atoms (2 axial + 1 equatorial). This gives T-shaped molecular geometry (the 2 axial F + 1 equatorial F form a T).
📌 ClF (AB): Linear, no lone pairs
📌 ClF₃ (AB₃): T-shaped, 2 lp on Cl — this question!
📌 ClF₅ (AB₅): Square pyramidal, 1 lp on Cl
📌 BrF₃ (AB₃): T-shaped, 2 lp on Br (same as ClF₃)
📌 BrF₅ (AB₅): Square pyramidal, 1 lp on Br
📌 IF₇ (AB₇): Pentagonal bipyramidal, 0 lp on I
📌 ICl (AB): Linear
📌 ICl₃ (AB₃): T-shaped, 2 lp on I
📌 Interhalogen compounds: between two different halogens, more electronegative halogen is terminal
In ideal T-shape (from trigonal bipyramidal): F(axial)−Cl−F(axial) = 180°, F(axial)−Cl−F(equatorial) = 90°. However, due to lone pair repulsions (lp-lp and lp-bp), the actual bond angles are slightly less than ideal: F(axial)−Cl−F(equatorial) ≈ 87·5° (slightly less than 90° due to 2 lone pairs pushing the bond pairs inward). This distortion from ideal geometry is a key feature explained by VSEPR theory.
Steric number = 5 → sp³d hybridisation. Cl uses one 3s, three 3p, and one 3d orbital to form 5 hybrid orbitals. Three hybrid orbitals form bonds with F atoms. Two hybrid orbitals hold lone pairs. The d orbital participation allows Cl to expand its octet (have more than 8 electrons around it) — this is possible for elements in Period 3 and beyond (they have available d orbitals). Period 2 elements (C, N, O, F) cannot expand octet — no available d orbitals.
📌 XeF₂: Steric No. 5, 3 lp, 2 bp → Linear (lp in equatorial)
📌 XeF₄: Steric No. 6, 2 lp, 4 bp → Square planar (lp on opposite sides)
📌 XeF₆: Steric No. 7, 1 lp, 6 bp → Distorted octahedral
📌 XeO₃: Steric No. 4, 1 lp, 3 bp → Trigonal pyramidal
📌 XeO₄: Steric No. 4, 0 lp, 4 bp → Tetrahedral
📌 XeOF₄: Steric No. 6, 1 lp, 5 bp → Square pyramidal
Linear (180°): CO₂, BeCl₂, CS₂, HCN, C₂H₂. Trigonal planar (120°): BF₃, BCl₃, SO₃, NO₃⁻, CO₃²⁻. Bent (~105° or ~120°): H₂O (104·5°), H₂S (92°), SO₂ (119·5°), NO₂ (134°). Tetrahedral (109·5°): CH₄, CCl₄, SiCl₄, NH₄⁺, PO₄³⁻. Trigonal pyramidal (~107°): NH₃ (107°), PCl₃, NF₃, H₃O⁺. T-shaped (~87·5°): ClF₃, BrF₃, ICl₃. Square planar (90°): XeF₄, [Pt(CN)₄]²⁻, [NiCl₄]²⁻ (d²sp³). Octahedral (90°): SF₆, [Co(NH₃)₆]³⁺.