# Hybridisation and Molecular Shapes

> Hybridisation and shapes for NEET Chemistry: steric number counting, sp to sp3d3 geometries and lone-pair shapes — see-saw, T-shape, square planar.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/hybridisation-shapes-neet
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Hybridisation and Molecular Shapes", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/hybridisation-shapes-neet

## Direct answer

Count regions of electron density around the central atom — sigma bonds plus lone pairs, with each multiple bond counting as one region — and both hybridisation and shape follow: two regions sp and linear 180°, three sp2 and trigonal planar 120°, four sp3 and tetrahedral 109.5°, five sp3d and trigonal bipyramidal, six sp3d2 and octahedral, seven sp3d3 as in IF7. Molecular shape ignores lone pairs while electron-pair geometry includes them: water's four sp3 pairs give a bent 104.5° molecule, ammonia's give pyramidal 107°, squeezed below 109.5° because lone pair-bond pair repulsion exceeds bond pair-bond pair. In trigonal bipyramids lone pairs always sit equatorial, producing the exam's beloved distorted shapes: see-saw SF4, T-shaped ClF3, linear XeF2.

## What you must remember

- **Steric number rule:** hybridisation = number of sigma bonds + number of lone pairs on the central atom; pi bonds never add regions, so CO2's two double bonds still mean sp.
- **Reference molecules:** sp — BeCl2, CO2, C2H2; sp2 — BF3, SO3, C2H4, NO3−; sp3 — CH4, NH3 (pyramidal, 107°), H2O (bent, 104.5°), NH4+; sp3d — PCl5; sp3d2 — SF6; sp3d3 — IF7 (72° and 90° angles).
- **Repulsion order:** lone pair-lone pair > lone pair-bond pair > bond pair-bond pair — the reason angles shrink from 109.5° (CH4) to 107° (NH3) to 104.5° (H2O).
- **TBP lone-pair rule:** lone pairs occupy equatorial positions (fewer 90° interactions), giving SF4 see-saw, ClF3 T-shaped (two equatorial lone pairs) and XeF2 linear (three equatorial lone pairs).
- **Octahedral derivatives:** XeF4 has two lone pairs trans to each other — square planar; BrF5 one lone pair — square pyramidal.
- **PCl5 asymmetry:** axial bonds (240 pm) are longer and weaker than equatorial (202 pm) in the solid and vapour — NCERT's quoted numbers.
- **I3− geometry:** two bond pairs plus three equatorial lone pairs on the central iodine — linear anion, a standard surprise question.

## From count to shape: xenon tetrafluoride and friends

Take XeF4. Total valence electrons: 8 + 4 × 7 = 36. Four Xe–F sigma bonds consume 8, each fluorine holds three lone pairs (24), leaving 4 on xenon — two lone pairs. Steric number 4 + 2 = 6: sp3d2, octahedral electron-pair geometry. The two lone pairs take opposite (trans) positions to minimise their mutual repulsion, and the molecular shape is square planar, all F–Xe–F angles 90° in one plane. Contrast BrF5, one lone pair short: square pyramidal, with the lone pair crowding the apex.

Now SF4. Six plus four sevens is 34 electrons; four bonds and one lone pair make steric number 5 — trigonal bipyramidal electron geometry. The lone pair claims an equatorial seat, and the molecule becomes a see-saw. ClF3 goes one further: three bonds plus two lone pairs, both equatorial, leaving a T-shape. Notice the pattern — every extra equatorial lone pair shaves a bond position off the equatorial plane while the two axial bonds persist. That single diagram, drawn once and remembered, answers half the shape questions NEET asks.

## Counting slips that mislabel hybridisation

Carbon dioxide is the gateway error: two double bonds are two regions, sp, linear — candidates who count four bonds write sp3. Graphite's carbons are sp2 and diamond's sp3, a materials-flavoured favourite. The second slip is reporting electron-pair geometry when the question asks molecular shape: water is bent, not tetrahedral; the tetrahedron belongs to its electron pairs. Third, the axial-equatorial distinction in PCl5 — axial bonds are the longer, weaker pair, and interconversion through pseudo-rotation is beyond the syllabus but the length difference is asked. Fourth, hybridisation belongs to the central atom's orbitals, so species like NH4+ (sp3) and I3− (sp3d on the central iodine) need the charge folded into the electron count before any counting begins.

## Frequently asked questions

### What are the hybridisation and shape of SF4?

sp3d, see-saw — the lone pair occupies an equatorial position of the trigonal bipyramid, distorting the shape.

### Why is XeF4 square planar rather than tetrahedral?

Its two lone pairs sit trans in an octahedral sp3d2 arrangement, leaving four bonds in one square plane.

### How many regions of electron density does each carbon of ethyne have?

Two — one sigma bond to hydrogen and one triple bond to the other carbon — so each is sp hybridised and linear.

### Why do lone pairs take equatorial positions in a trigonal bipyramid?

Equatorial placement keeps lone pairs at 90° to only two bonds instead of three, minimising repulsion.

### Which bonds of PCl5 are longer and why?

The axial bonds (240 pm against 202 pm equatorial), because each axial pair suffers more 90° repulsions than an equatorial bond.