VSEPR Theory: Predict Any Molecular Shape in Four Steps for NEET and JEE

Why is CO₂ a straight line while H₂O is bent, when both have the formula pattern AB₂? And why does NH₃ sit at about 107° when a tetrahedron says 109.5°? VSEPR theory answers all of this with one idea: electron clouds repel each other, so they spread out as far as they can. This concept matters for both NEET and JEE, mostly under Chemical Bonding in Class 11.

Quick Summary

• VSEPR = Valence Shell Electron Pair Repulsion. Count electron domains (bond pairs + lone pairs) around the central atom.

• The number of domains fixes the electron geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.

• Lone pairs occupy space but are invisible in the final molecular shape, which is named from the atoms only.

• Repulsion order: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.

Why electron pairs spread out

Take balloons tied together at a knot. Two balloons point opposite ways (180°), three form a flat triangle (120°), four form a tetrahedron (109.5°). Electron pairs around a central atom behave the same way because each pair is a negatively charged cloud and like charges repel. The shape that keeps all clouds farthest apart is the lowest-energy arrangement, so that is the shape the molecule adopts.

The key move is counting domains. A single bond, a double bond, a triple bond and a lone pair each count as one domain, because a multiple bond is one region of electron density between two atoms. That is why CO₂ (O=C=O) has two domains and is linear.

Step-by-step method

1. Pick the central atom and count its valence electrons (V).

2. Count the atoms bonded to it (B). For a neutral molecule with single-bonded monovalent or halogen atoms, bond pairs = B.

3. Lone pairs L = (V − B) ÷ 2 for such molecules. For ions, add electrons for a negative charge and subtract for a positive one before dividing.

4. Domains = B + L. Read off the electron geometry from the count.

5. Remove the lone pairs from the picture to name the molecular shape.

Shapes you must know

• 2 domains: BeCl₂, CO₂ – linear, 180°.

• 3 domains: BF₃ (0 lone pairs) – trigonal planar, 120°; SO₂ (1 lone pair) – bent, about 119°.

• 4 domains: CH₄ – tetrahedral, 109.5°; NH₃ (1 lone pair) – pyramidal, about 107°; H₂O (2 lone pairs) – bent, about 104.5°.

• 5 domains: PCl₅ – trigonal bipyramidal; SF₄ – see-saw; ClF₃ – T-shaped; XeF₂ – linear.

• 6 domains: SF₆ – octahedral; BrF₅ – square pyramidal; XeF₄ – square planar.

Worked example: the shape of SF₄

Sulfur has 6 valence electrons (V = 6). It bonds to 4 fluorine atoms (B = 4).

Lone pairs L = (6 − 4) ÷ 2 = 1.

Domains = 4 + 1 = 5, so the electron geometry is trigonal bipyramidal, with 3 equatorial positions (120° apart) and 2 axial positions (90° to the equator).

Where does the lone pair go? A lone pair repels more strongly than a bond pair, so it takes the position with the fewest close neighbours at 90°. An equatorial lone pair has two neighbours at 90°; an axial one would have three. So the lone pair goes equatorial.

Removing it leaves four fluorines in a shape like a see-saw. Because of lone-pair repulsion, the axial F–S–F angle is slightly less than 180° and the equatorial F–S–F angle is slightly less than 120°. Lone pairs always take equatorial positions in trigonal bipyramidal geometry.

Check with H₂O in the same way: V = 6, B = 2, L = 2, domains = 4 → tetrahedral electron geometry, bent molecular shape. The two lone pairs squeeze the H–O–H angle from 109.5° down to about 104.5°.

Mistakes students actually make

• Confusing electron geometry with molecular shape. H₂O is tetrahedral in electron geometry but bent in shape. Read the question for which one is asked.

• Counting a double bond as two domains. It is one domain. This gives the wrong answer for CO₂, SO₂ and SO₃.

• Forgetting the charge on ions. For NH₄⁺ subtract one electron (V = 4); for ClO₃⁻ add one electron before halving.

• Putting the lone pair axial in 5-domain species such as SF₄, ClF₃ and XeF₂.

• Assuming an ideal angle for lone-pair molecules. NH₃ is not 109.5°, because the lone pair compresses the H–N–H angles.

• Treating shape as polarity. A symmetric shape (CO₂, BF₃, XeF₄) can cancel individual bond dipoles even when each bond is polar.

How it is asked in NEET and JEE

• "Identify the shape of XeF₄ / ClF₃ / SF₄ / I₃⁻": a direct domain-counting problem.

• "Arrange NH₃, H₂O and CH₄ in order of bond angle": answer CH₄ > NH₃ > H₂O, by lone-pair repulsion.

• "Which species has zero dipole moment?": needs shape first, then symmetry.

• Matching lists of molecules to shapes, or finding the number of lone pairs on the central atom.

• Comparing angles in a series, such as NH₃ vs NF₃ or H₂O vs H₂S, which adds electronegativity to the argument.

Pro Tip: For the angle in AX₃ and AX₂ molecules with the same lone-pair count, the more electronegative the surrounding atoms, the more they pull bond pairs away from the central atom, so the bond pairs repel each other less and the angle shrinks. That is why NF₃ (about 102°) has a smaller angle than NH₃ (about 107°). When the central atom changes down a group (NH₃ to PH₃), the angle falls sharply because bond pairs sit farther from the nucleus and are repelled less.

Frequently Asked Questions

Does VSEPR work for every molecule?

It works very well for main-group molecules with a clear central atom. It is less reliable for transition-metal complexes, where d-orbital effects matter. For NEET and JEE, treat it as the standard tool for p-block species.

Why do lone pairs repel more than bond pairs?

A bond pair is held between two nuclei, so it is pulled into a narrower region. A lone pair belongs to only one nucleus, so its cloud spreads out and occupies more space. This is why lone pairs shrink bond angles.

How do I find the shape of an ion like ICl₄⁻?

Iodine has 7 valence electrons, and the negative charge adds one, giving 8. With 4 bonds, L = (8 − 4) ÷ 2 = 2. That is 6 domains, so the electron geometry is octahedral. The two lone pairs sit opposite each other, giving a square planar shape.

Why is the bond angle in H₂O less than in NH₃?

H₂O has two lone pairs and NH₃ has one. More lone-pair repulsion pushes the bond pairs closer together, so the angle drops from about 107° in NH₃ to about 104.5° in H₂O.

Is the shape of a molecule the same as its hybridisation?

No, but they are linked. The number of domains decides the hybridisation (4 domains means sp³, for example), while the lone pairs decide the final shape. NH₃ and CH₄ are both sp³ yet have different shapes.

Stuck on a tricky shape such as XeOF₄ or I₃⁻? Drop your doubt in the comments, and share this with a fellow aspirant who still mixes up electron geometry and molecular shape.