Geometries (With Lone Pairs)

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the three-dimensional shape of a molecule by minimising repulsions between all electron pairs — both bonding pairs and lone pairs — around a central atom. While bonding pairs are shared between two nuclei and are relatively compact, lone pairs occupy more space because they are held by only one nucleus and spread out more broadly. This difference in repulsion strength is the key to understanding why molecules with lone pairs adopt geometries that deviate from the 'ideal' shapes seen when every electron domain is a bond. The geometries covered in this article all arise from electron-pair geometries that you might already recognise, but the molecular geometry — the shape described only by the positions of the atoms — differs once lone pairs are accounted for. For example, four electron domains around a central atom give a tetrahedral electron geometry, but if one domain is a lone pair the molecular shape is trigonal pyramidal (as in $\text{NH}_3$ ), and if two are lone pairs the shape is bent (as in $\text{H}_2\text{O}$ ). The greater repulsive 'push' of lone pairs compresses bond angles below their ideal values. Moving to five and six electron domains introduces even richer geometry. With five domains, lone pairs preferentially occupy equatorial positions to minimise the number of 90° interactions, giving seesaw (one lone pair, $\text{SF}_4$ ) and T-shaped (two lone pairs, $\text{ClF}_3$ ) geometries. Six electron domains with two lone pairs placed opposite each other produce the square planar geometry, famously seen in $\text{XeF}_4$ and in important transition-metal complexes. Understanding these shapes is essential because molecular geometry governs polarity, reactivity, and physical properties.

Key takeaways

Count All Electron Domains
Identify the Electron-Pair Geometry
Apply Lone-Pair Repulsion Rules
Name the Molecular Geometry
Assess Polarity

Count All Electron Domains

Draw the Lewis structure of the molecule. Count every electron domain around the central atom: each single bond, double bond, triple bond, and lone pair counts as one domain. For $\text{H}_2\text{O}$ : O has 2 bonds + 2 lone pairs = 4 domains. For $\text{NH}_3$ : N has 3 bonds + 1 lone pair = 4 domains. For $\text{SF}_4$ : S has 4 bonds + 1 lone pair = 5 domains. For $\text{ClF}_3$ : Cl has 3 bonds + 2 lone pairs = 5 domains. For $\text{XeF}_4$ : Xe has 4 bonds + 2 lone pairs = 6 domains.

Identify the Electron-Pair Geometry

The electron-pair geometry depends only on the total number of domains: 2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral. All five molecules in this article fall into the tetrahedral (4 domains), trigonal bipyramidal (5 domains), or octahedral (6 domains) categories.

Apply Lone-Pair Repulsion Rules

Repulsion strength follows the order: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. In a trigonal bipyramidal arrangement (5 domains), the equatorial positions have 120° separation while axial positions have only 90° contacts with the equatorial set. Lone pairs are therefore always placed equatorially to minimise the number of unfavourable 90° repulsions. In an octahedral arrangement (6 domains), lone pairs are placed trans (opposite) to each other so their 180° separation minimises repulsion.

Name the Molecular Geometry

Ignore lone pairs when naming the shape — only atom positions matter. Bent ( $\text{H}_2\text{O}$ ): tetrahedral electron geometry, 2 lone pairs, bond angle ≈ 104.5° (reduced from 109.5° by lone-pair compression). Trigonal pyramidal ( $\text{NH}_3$ ): tetrahedral electron geometry, 1 lone pair, bond angle ≈ 107° (reduced from 109.5°). Seesaw ( $\text{SF}_4$ ): trigonal bipyramidal electron geometry, 1 equatorial lone pair, axial F–S–F ≈ 173°, equatorial F–S–F ≈ 102°. T-shaped ( $\text{ClF}_3$ ): trigonal bipyramidal electron geometry, 2 equatorial lone pairs, F–Cl–F (axial) ≈ 87.5°. Square planar ( $\text{XeF}_4$ ): octahedral electron geometry, 2 trans lone pairs, all F–Xe–F angles = 90°.

Assess Polarity

Once the geometry is known, check whether bond dipoles cancel. In $\text{H}_2\text{O}$ the bent shape means the two O–H dipoles do not cancel → polar molecule. In $\text{XeF}_4$ the four Xe–F dipoles are arranged symmetrically in a plane and cancel → nonpolar molecule, despite each bond being polar. This demonstrates why geometry — not just electronegativity — determines molecular polarity.

The Key Equation

\text{Steric Number} = (\text{bonding domains}) + (\text{lone pairs})

SN — Steric Number (dimensionless): Total number of electron domains (bonds + lone pairs) around the central atom; sets the electron-pair geometry.
BP — Bonding Domains (dimensionless): Each single, double, or triple bond counts as one bonding domain regardless of bond order.
LP — Lone Pairs (dimensionless): Non-bonding electron pairs on the central atom; exert greater repulsion than bonding pairs and distort bond angles.

Worked Example

Problem. Determine the molecular geometry and predict whether $\text{SF}_4$ is polar or nonpolar. Sulfur is the central atom; its Lewis structure shows S forming four single bonds to F with one lone pair remaining on S.

\text{Seesaw geometry; } \mu \neq 0 \text{ (polar)}

Answer. SF₄ has a seesaw (C₂ᵥ) molecular geometry and is a polar molecule.

Figure

Diagram

Origins · 1 minRonald James Gillespie and how this was discovered

Ronald James Gillespie

1924–2021 · British-Canadian

Ronald Gillespie, working with Ronald Nyholm, formalised VSEPR theory in 1957. Their landmark paper provided chemists with a simple, intuitive model to predict molecular geometry from Lewis structures alone, without requiring quantum mechanical calculations. Gillespie later refined the model to incorporate the effects of multiple bonds and electronegative substituents on bond angles, and spent decades at McMaster University developing educational frameworks for general chemistry. Gillespie was passionate about science education throughout his 60-year career and wrote widely used undergraduate textbooks; he argued that VSEPR, despite its simplicity, remains one of the most powerful predictive tools in all of chemistry.

VSEPR Geometries (Without Lone Pairs) — Provides the baseline ideal geometries — linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral — from which lone-pair geometries are derived.
Lewis Structures and Formal Charge — Correct Lewis structures are the starting point for all VSEPR predictions; the number and type of electron domains are read directly from the Lewis diagram.
Molecular Polarity and Dipole Moments — Molecular geometry determines whether bond dipoles cancel; lone-pair geometries like bent and pyramidal produce polar molecules while square planar produces a nonpolar molecule.
Orbital Hybridisation (sp, sp², sp³) — Hybridisation provides a quantum mechanical basis for the electron domains assumed by VSEPR; sp³ hybridisation underlies the tetrahedral electron geometry of water and ammonia.
Hydrogen Bonding — The bent geometry of water, arising from its two lone pairs, positions the lone pairs perfectly to form hydrogen bonds, explaining water's unusually high boiling point.