VSEPR Molecular Shapes — Chemistry Mnemonic
Target: mnemonic for VSEPR theory molecular geometry shapes
Why is this hard to memorize?
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry based on electron pair count around the central atom. For NEET and JEE, you need to instantly map electron pair arrangements to shapes: 2 pairs = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral. Then account for lone pairs to get the actual shape (e.g., 4 pairs with 1 lone pair = trigonal pyramidal like NH₃). This topic connects to hybridization, bond angles, and dipole moments — it's tested every year.
Classic mnemonics you should know
"2-Linear, 3-TriP, 4-Tetra, 5-TBP, 6-Octa — "23456 = LTTTO""
Map numbers to shapes: 2=Linear, 3=Trigonal Planar, 4=Tetrahedral, 5=Trigonal Bipyramidal, 6=Octahedral. "LTTTO" is the shape sequence.
"4 pairs: 0 LP=Tetra(CH₄), 1 LP=Pyramid(NH₃), 2 LP=Bent(H₂O)"
Start from the ideal shape and "remove" atoms to add lone pairs. Each removal changes the shape name: tetrahedral → pyramidal → bent. Same logic for 5 and 6 pairs.
"BeCl₂=Line, BF₃=Flat, CH₄=3D, PCl₅=Star, SF₆=Box"
One example molecule per shape: BeCl₂(linear), BF₃(trigonal planar), CH₄(tetrahedral), PCl₅(trigonal bipyramidal), SF₆(octahedral). Memorize these five and derive everything else.
The complete list
- Linear (2 pairs, 180°)
- Trigonal planar (3 pairs, 120°)
- Tetrahedral (4 pairs, 109.5°)
- Trigonal pyramidal (3 BP + 1 LP)
- Bent/V-shape (2 BP + 2 LP)
- Trigonal bipyramidal (5 pairs)
- See-saw (4 BP + 1 LP)
- T-shape (3 BP + 2 LP)
- Octahedral (6 pairs, 90°)
- Square pyramidal (5 BP + 1 LP)
- Square planar (4 BP + 2 LP)
Frequently asked questions
How do I quickly determine molecular shape using VSEPR?
Step 1: Count total electron pairs around central atom (bonding + lone). Step 2: Use the number to get the electron geometry (2=linear, 3=trigonal, 4=tetra, 5=TBP, 6=octa). Step 3: Subtract lone pairs to get the molecular shape name. Step 4: Predict approximate bond angle (lone pairs compress angles by ~2-3° each).
Why does water have a 104.5° angle instead of 109.5°?
Water has 4 electron pairs (2 bonding, 2 lone). The electron geometry is tetrahedral (109.5°), but the two lone pairs repel more strongly than bonding pairs, compressing the H–O–H angle from 109.5° to 104.5°. Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair repulsion.
How do I remember where lone pairs go in trigonal bipyramidal?
Lone pairs always go in equatorial positions in TBP because equatorial positions have only two 90° interactions (with axial bonds), while axial positions have three. This minimizes repulsion. So SF₄ (1 LP) = see-saw, ClF₃ (2 LP) = T-shape, XeF₂ (3 LP) = linear.
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