Question
Predict the shape and bond angle of , , and using VSEPR theory. All three have 5 electron pairs around the central atom — why do they have different shapes?
(JEE Main + NEET repeated pattern)
Solution — Step by Step
For each molecule, find the total electron pairs (bonding pairs + lone pairs):
| Molecule | Bonding pairs (BP) | Lone pairs (LP) | Total pairs |
|---|---|---|---|
| 4 | 1 | 5 | |
| 3 | 2 | 5 | |
| 2 | 3 | 5 |
All three have 5 electron pairs — so the electron pair geometry is trigonal bipyramidal for all three.
Lone pairs occupy more space than bonding pairs. In a trigonal bipyramidal arrangement, equatorial positions have more room (120 degree angles) compared to axial positions (90 degree angles). So lone pairs always go equatorial first.
This is the key VSEPR rule that determines the final shape.
-
SF₄ (1 LP equatorial): The 4 bonding pairs form a see-saw (or distorted tetrahedral) shape. Bond angles are approximately 117 degrees (equatorial) and 187 degrees (axial, bent away).
-
ClF₃ (2 LP equatorial): The 3 bonding pairs form a T-shape. Bond angles are slightly less than 90 degrees due to LP-BP repulsion.
-
XeF₂ (3 LP equatorial): The 2 bonding pairs sit axial — giving a linear shape with a 180 degree bond angle.
VSEPR Shape Prediction Flowchart
flowchart TD
A["Count total electron pairs around central atom"] --> B{"How many total pairs?"}
B -->|"2"| C["Linear — 180°"]
B -->|"3"| D{"Lone pairs?"}
B -->|"4"| E{"Lone pairs?"}
B -->|"5"| F{"Lone pairs?"}
B -->|"6"| G{"Lone pairs?"}
D -->|"0 LP"| D1["Trigonal planar — 120°"]
D -->|"1 LP"| D2["Bent — ~117°"]
E -->|"0 LP"| E1["Tetrahedral — 109.5°"]
E -->|"1 LP"| E2["Trigonal pyramidal — ~107°"]
E -->|"2 LP"| E3["Bent — ~104.5°"]
F -->|"0 LP"| F1["Trigonal bipyramidal"]
F -->|"1 LP"| F2["See-saw"]
F -->|"2 LP"| F3["T-shape"]
F -->|"3 LP"| F4["Linear"]
G -->|"0 LP"| G1["Octahedral — 90°"]
G -->|"1 LP"| G2["Square pyramidal"]
G -->|"2 LP"| G3["Square planar"]Why This Works
VSEPR (Valence Shell Electron Pair Repulsion) theory rests on one idea: electron pairs around a central atom repel each other and arrange themselves to maximise distance.
The repulsion order is: LP-LP > LP-BP > BP-BP. Lone pairs spread out more because they are held by only one nucleus, so they push bonding pairs closer together. This is why actual bond angles are always slightly less than ideal angles.
The same total electron pair count (5 in our case) gives different molecular shapes because the number of lone pairs changes. The electron pair geometry stays trigonal bipyramidal — but the molecular geometry (what we actually observe) depends on where the atoms sit.
Common Mistake
The biggest error: confusing electron pair geometry with molecular geometry. Water has 4 electron pairs (tetrahedral electron pair geometry) but only 2 bonding pairs — so its molecular geometry is bent, not tetrahedral. NEET and JEE questions specifically test this distinction. Always report the molecular shape, not the electron pair arrangement.
For quick VSEPR predictions in exams: draw the Lewis structure, count regions of electron density (double and triple bonds count as ONE region), subtract lone pairs from total to get bond pairs, then match to the flowchart above. Takes under 30 seconds with practice.