VSEPR for Main-Group Compounds

Introduction

The same number of bonded atoms can produce different shapes when the central atom has lone pairs. VSEPR organizes this reasoning by counting regions of electron density, then distinguishing the arrangement of all regions from the arrangement of atoms alone. The model is especially useful for small main-group molecules and ions; it predicts approximate, not exact, angles.

Core explanation

First draw a reasonable Lewis structure. Each single, double or triple bond to a terminal atom counts as one electron domain for VSEPR, while each lone pair on the central atom counts as another. Two domains tend to arrange linearly at 180°. Three tend to form a trigonal plane at 120°. Four tend toward a tetrahedron at about 109.5°. Five give a trigonal-bipyramidal domain arrangement, and six an octahedral one. These are domain geometries, not automatically the final molecular shapes.

Methane, CH₄, has four bonding domains and no central lone pair, so both domain and molecular geometry are tetrahedral. Ammonia, NH₃, has three N–H bonds plus one N lone pair; its domain arrangement is tetrahedral, but its molecular shape is trigonal pyramidal. Water has two O–H bonds plus two O lone pairs; its domain arrangement is tetrahedral, but its molecular shape is bent. Lone pairs generally repel bonding domains strongly and can compress bond angles: NH₃'s H–N–H angle is less than 109.5°, and water's H–O–H angle is smaller still.

For three domains, BF₃ has three bonds and no lone pair, giving trigonal planar atoms. SO₂ has two bonding domains and a lone-pair domain around sulfur in a simple VSEPR account, so the electron-domain geometry is trigonal planar but the molecule is bent. A multiple bond counts once for geometry even though it contains more than one shared electron pair; it may repel somewhat differently from a single bond. Thus exact measured angles require more than counting domains.

For five domains, PCl₅ has a trigonal-bipyramidal molecular shape with three equatorial and two axial positions. If one domain is a lone pair, as in SF₄, the molecular shape is seesaw. Lone pairs preferentially occupy equatorial positions in a trigonal bipyramid because that arrangement reduces close 90° interactions. For six domains, SF₆ is octahedral; XeF₄ has four bonds and two opposite lone pairs, so its molecular shape is square planar. This example shows why “four fluorine atoms” does not imply tetrahedral shape.

VSEPR is a geometric model based on electron-density repulsions, not a proof that electrons sit in rigid balloons. Other nuclear and electronic interactions also affect geometry. Transition-metal complexes require additional ligand-field considerations, so do not use a simple main-group VSEPR table to choose between square-planar and tetrahedral metal complexes without more information. OpenStax Chemistry 2e explains and illustrates these examples at https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity.

Step-by-step reasoning

1. Count valence electrons and draw the Lewis structure. 2. Count central bonding domains, treating each multiple bond as one region. 3. Add central lone-pair domains and find total domain number. 4. Assign the domain geometry, then remove lone-pair positions to name molecular shape. 5. State approximate angle and any lone-pair distortion or model limitation.

Visual explanation

Draw a four-domain tetrahedron in three versions: CH₄ with four atom corners, NH₃ with one corner marked lone pair, and H₂O with two corners marked lone pairs. The domain framework stays similar while the visible arrangement of atoms changes from tetrahedral to pyramidal to bent.

Real-world analogy

Four seats arranged around a table remain in the same positions even if one or two seats are occupied by invisible bags. The occupied person positions change, while the full seat pattern does not. Lone pairs are the unseen positions that affect where bonded atoms can sit.

Real-world example

Water's bent geometry creates a permanent molecular dipole, helping explain its strong intermolecular attractions. Carbon dioxide also has polar bonds, but its two domains make a linear symmetric molecule with canceling dipoles. Shape predictions therefore help connect bonding to bulk behaviour.

Why?

Why does a double bond count as one domain? The electron density of both shared pairs lies in the same general direction between the central and terminal atoms. VSEPR counts regions of density around the centre rather than individual bond lines in a Lewis diagram.

Common misconception

“Four bonds always mean tetrahedral” fails for XeF₄. Xenon has four bonding domains and two lone pairs, six domains in total; the lone pairs occupy opposite octahedral positions, leaving four fluorines in a square plane.

Worked example

Predict SO₂. A Lewis/resonance representation gives sulfur two S–O bonding regions and one lone-pair region, total three domains. The domain arrangement is trigonal planar. Only two positions contain oxygen nuclei, so the molecular geometry is bent, with an O–S–O angle near but not exactly 120°. Its bent shape means S–O bond dipoles do not cancel.

Quick check

1. What are the domain geometry and molecular shape of NH₃? Answer: Four domains give tetrahedral electron-domain geometry; three N–H bonds and one lone pair give trigonal-pyramidal molecular shape.

Exam focus

Show the bond and lone-pair counts before naming shape. Do not confuse domain geometry with the arrangement of atoms. Give an approximate ideal angle, then note distortion when lone pairs or multiple bonds change repulsion.

Advanced insight

Electron-density domains are an empirical simplification. In a real molecule, bond-angle deviations depend on bond polarity, ligand size, multiple-bond character and electronic delocalization. VSEPR performs well for many main-group examples but should be checked against measured or higher-level predicted structures for subtle cases.

Summary

VSEPR counts bonding and lone-pair domains to estimate their arrangement; molecular geometry names only atom positions. Two, three, four, five and six domains give familiar ideal frameworks. Lone pairs can change shape and angle even when the number of bonded atoms stays the same.

Practice questions

1. What molecular shape does H₂O have, and how many central domains? Answer: Bent, with four domains: two O–H bonds and two lone pairs. 2. Why is BF₃ trigonal planar? Answer: Boron has three bonding domains and no central lone pair in its simple structure. 3. What shape is XeF₄ under VSEPR? Answer: Square planar, because four bonds and two opposite lone pairs occupy six octahedral domain positions.