The trap that costs marks: You see NH₃ and count three bonds, so you call the geometry "trigonal planar." But that lone pair on nitrogen is invisible in the formula and very visible in the shape. VSEPR counts all electron pairs around the central atom — bonding and lone — to determine the electron-pair geometry first, and only then names the molecular shape from the atom positions alone.
VSEPR core idea. Valence Shell Electron Pair Repulsion theory states that electron pairs around a central atom arrange themselves to minimise repulsion (NCERT Class 11 Chemistry Chapter 4, page 14). The number of electron pairs — the steric number (SN) — fixes the electron-pair geometry:
| SN | Electron-pair geometry | Example with 0 lone pairs |
|---|---|---|
| 2 | Linear | BeCl₂ |
| 3 | Trigonal planar | BF₃ |
| 4 | Tetrahedral | CH₄ |
| 5 | Trigonal bipyramidal | PCl₅ |
| 6 | Octahedral | SF₆ |
Lone pairs distort. Lone-pair–bond-pair repulsion is stronger than bond-pair–bond-pair repulsion (NCERT Class 11 Chemistry Chapter 4, page 16). This compresses bond angles below the ideal values. So while NH₃ has SN = 4 (tetrahedral electron geometry), its molecular shape is trigonal pyramidal with bond angle ~107° instead of 109.5°. H₂O with SN = 4 but two lone pairs is bent at ~104.5°.
NEET relevance. Questions typically give you a molecular formula and ask for the shape or bond angle comparison. The high-frequency trap: forgetting to include lone pairs in the steric number. Always compute SN = (bonding pairs) + (lone pairs) before naming the geometry.
Watch-out: Electron-pair geometry ≠ molecular shape when lone pairs are present. NEET distractors routinely offer the electron-pair geometry name as a wrong option for the molecular shape.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
According to VSEPR theory, electron pairs around a central atom arrange themselves to:
In VSEPR theory, which type of repulsion is the strongest?
What is the electron-pair geometry of a molecule with a steric number of 4 and no lone pairs on the central atom?
The steric number of the central atom in H₂O is:
NH₃ has a bond angle of approximately 107° instead of the ideal tetrahedral angle of 109.5°. This compression is because:
Which of the following molecules has a bent molecular shape?
A student predicts that NH₃ is trigonal planar because nitrogen forms three bonds. The error in this reasoning is:
Arrange the following in order of decreasing bond angle: CH₄, NH₃, H₂O.
Pattern: Predict molecular geometry from VSEPR steric number and lone pairs (P.CHE.U03.VSEPR_GEOMETRY; observed in NEET 2022, 2024, 2025).
Given
- Molecule: ClF₃ - Chlorine is the central atom (less electronegative) - Cl has 7 valence electrons; each F has 7 valence electrons
Required
- Molecular shape and approximate bond angle of ClF₃
Concept
VSEPR theory: compute steric number (SN = bonding pairs + lone pairs on central atom), determine electron-pair geometry, then identify molecular shape from atom positions only.
Formula
SN = (number of bonding pairs) + (number of lone pairs on central atom)
Substitution
Total valence electrons = 7 + 3(7) = 28. Three Cl–F bonds use 6 electrons. Remaining on Cl: 7 − 3 = 4 electrons (after formal assignment) → but let's count systematically. Place 3 bond pairs (6 e⁻). Remaining: 28 − 6 = 22 e⁻. Each F gets 3 lone pairs: 3 × 6 = 18 e⁻ used. Remaining on Cl: 22 − 18 = 4 e⁻ = 2 lone pairs. SN = 3 (bond pairs) + 2 (lone pairs) = 5.
Calculation
SN = 5 → electron-pair geometry is trigonal bipyramidal. With 2 lone pairs: lone pairs occupy equatorial positions (to minimise 90° lp–bp repulsions). Molecular shape: T-shaped. Bond angle: approximately 87.5° (less than 90° due to lp–bp compression).
Final answer
ClF₃ has a T-shaped molecular geometry with bond angles of approximately 87.5°. Note: the counting numbers (3 bond pairs, 2 lone pairs, 28 total electrons) are exact integers and do not affect significant-figure considerations.
Common trap
Forgetting the 2 lone pairs on Cl and assigning SN = 3 → trigonal planar. This is the same lone-pair omission trap seen in NH₃ and H₂O. Always count ALL electron pairs on the central atom.
Similar NEET-style question
"Predict the molecular shape of XeF₂. Given: Xe has 8 valence electrons." (Answer: SN = 5, 3 lone pairs + 2 bond pairs → linear.) ---
Electron pairs (bonding + lone) around central atom arrange to minimize repulsion. Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair. Geometry from steric number = bond pairs + lone pairs.
-- NCERT Class 11 Chemistry, Ch. 4, p. 14AB₂: linear (180°). AB₃: trigonal planar (120°). AB₄: tetrahedral (109.5°). AB₅: trigonal bipyramidal. AB₆: octahedral (90°). Lone pairs distort: AB₃E pyramidal, AB₂E₂ bent.
-- NCERT Class 11 Chemistry, Ch. 4, p. 16Higher bond order: shorter, stronger bond. N₂: BO=3, O₂: BO=2, F₂: BO=1.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N_b | bonding electrons | - |
| N_a | antibonding electrons | - |
| BO | bond order | - |
Product of charge magnitude and bond length. SI: C·m. Common: Debye (1 D = 3.336e-30 C·m).
| Symbol | Quantity | SI Unit |
|---|---|---|
| q | charge | C |
| d | bond length | m |
| mu | dipole moment | C*m or D |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student counts only bonded atoms when assigning hybridization. Lone pairs count toward steric number too. Steric number = bond pairs + lone pairs → hybridization.
Molecule with central atom having lone pairs (e.g., NH₃: 3 bonds + 1 lp = 4 = sp³; H₂O: 2+2 = 4 = sp³).
Steric number formula: SN = (bond pairs) + (lone pairs). SN=2: sp; SN=3: sp²; SN=4: sp³; SN=5: sp³d; SN=6: sp³d². Lone pairs distort but still count.
Root cause: concept gap
Steric number = bond pairs + lone pairs. NH3: SN=4 → sp³ (despite trigonal pyramidal shape).
Root cause: concept gap
Memorise the ordering switch at N2/O2 boundary. Affects bond order and magnetic property predictions.
Root cause: formula misuse
BO = (Nb - Na)/2. Antibonding electrons subtract. F2: 8 bonding, 6 antibonding → BO = 1.
14 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
The element expected to form largest ion to achieve the nearest noble gas configuration is
Which statement regarding polymers is not correct?
Amongst the following which one will have maximum ‘lone pair - lone pair’ electron repulsions?
In one molal solution that contains 0.5 mole of a solute, there is
Which of the following molecules is non-polar in nature?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
ignores lone pairs
Counts only bonding pairs
forgets anti bonding
Counts only bonding electrons
ignores lp bp distortion
Doesn't adjust for lone-pair repulsion
Test yourself on this topic with real past-paper questions:
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