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Molecular Geometry

Introduction

Molecular geometry is the three-dimensional arrangement of atoms in a molecule or polyatomic ion. At Grades 11–13, you should be able to move from a Lewis structure to a defensible prediction of shape, bond angles, and often molecular polarity. Geometry matters because the spatial arrangement of atoms influences physical properties, intermolecular attractions, reactivity, spectroscopy, and the way molecules interact in biological and technological systems.

This aiMOOC focuses on the Valence Shell Electron Pair Repulsion model, usually shortened to VSEPR. VSEPR is a predictive model: regions of electron density around a central atom arrange themselves to reduce repulsions. It is highly useful for many main-group molecules, but it is not a complete quantum-mechanical theory of bonding.

The diagram gives you an overview of common VSEPR shapes. Use it as a map rather than as a list to memorize: the key skill is to determine the number of electron domains and lone pairs, then reason from them to the molecular shape.

From Lewis Structures to Three-Dimensional Shape

Electron Domains

Start with a correct Lewis structure. Around a chosen central atom, count each region of electron density as one electron domain.

Single bond: One electron domain.
Double bond: One electron domain.
Triple bond: One electron domain.
Lone pair: One electron domain.

A multiple bond contains more electron density than a single bond, but for the first VSEPR count it still occupies one domain. The total number of domains is sometimes called the steric number in elementary VSEPR work.

Electron-Domain Geometry and Molecular Geometry

These two ideas must be kept separate. Electron-domain geometry describes the arrangement of all electron domains around the central atom, including lone pairs. Molecular geometry describes the arrangement of the bonded atoms and does not show lone pairs as vertices of the final shape.

For example, ammonia has four electron domains around nitrogen: three N–H bonding domains and one lone pair. Its electron-domain geometry is tetrahedral, but its molecular geometry is trigonal pyramidal.

AXE Notation

A compact way to organize VSEPR reasoning is AXE notation. A is the central atom, X is the number of atoms bonded to the central atom, and E is the number of lone pairs on the central atom. For example, CH4 is AX4, NH3 is AX3E, and H2O is AX2E2.

AXE notation is a bookkeeping tool. It does not replace the Lewis structure, because you still need the Lewis structure to determine the numbers of bonded atoms and lone pairs correctly.

The Five Basic Electron-Domain Geometries

Two and Three Electron Domains

With two electron domains, the domains point in opposite directions. The ideal geometry is linear with an angle of 180°. Carbon dioxide is a classic AX2 example.

With three electron domains, the ideal arrangement is trigonal planar. The domains lie in one plane with ideal angles of 120°. Boron trifluoride is an AX3 example.

Four Electron Domains

Four electron domains produce a tetrahedral electron-domain geometry. The ideal angle is about 109.5°. Methane is AX4, so both its electron-domain geometry and molecular geometry are tetrahedral.

If one of the four domains is a lone pair, as in AX3E ammonia, the molecular shape becomes trigonal pyramidal. If two domains are lone pairs, as in AX2E2 water, the molecular shape becomes bent.

The H–N–H angle in ammonia is about 107°, and the H–O–H angle in water is about 104.5°. These are smaller than the ideal tetrahedral angle because lone pairs repel neighboring electron domains more strongly than bonding pairs in the VSEPR model.

Datei:SP 5.2 Tetrahedral Geometry.webm

Five Electron Domains

Five electron domains form a trigonal bipyramidal electron-domain geometry. Three equatorial positions lie 120° apart in one plane, while two axial positions are perpendicular to that plane. Axial–equatorial angles are 90°, and the two axial positions are 180° apart.

Phosphorus pentachloride in the gas phase is a standard AX5 example.

When lone pairs occur in a trigonal bipyramidal arrangement, they preferentially occupy equatorial positions because this reduces the number of 90° interactions. Important derived molecular shapes are seesaw for AX4E, T-shaped for AX3E2, and linear for AX2E3.

Six Electron Domains

Six electron domains give an octahedral electron-domain geometry. Adjacent domains are 90° apart and opposite domains are 180° apart. Sulfur hexafluoride is an AX6 example.

Removing one bonded atom position and replacing it with a lone pair gives an AX5E square pyramidal molecular shape. With two lone pairs opposite each other, AX4E2 becomes square planar.

Repulsions and Bond-Angle Distortions

The simplest VSEPR picture treats electron domains as regions that compete for space around the central atom. A useful qualitative order is:

lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion

This ordering helps explain why lone pairs often compress adjacent bond angles. Multiple bonds also tend to occupy more angular space than single bonds, so real bond angles can deviate from the ideal values.

Do not treat ideal angles as exact measurements for every molecule. VSEPR predicts a geometric framework; actual angles depend on factors such as the identities of the atoms, bond multiplicity, lone pairs, and the electronic structure of the molecule.

A Reliable VSEPR Workflow

For a main-group molecule or ion, use the following reasoning sequence:

Lewis structure: Draw the valence-electron structure and verify the electron count.
Central atom: Identify the atom whose local geometry you want to predict.
Electron domain: Count bonding regions and lone pairs around that atom.
Electron-domain geometry: Match the domain count to the basic geometry.
Molecular geometry: Ignore lone-pair positions when naming the arrangement of atoms.
Bond angle: State the ideal angle and, where appropriate, predict compression or expansion.
Molecular polarity: Combine bond dipoles as vectors and consider whether the geometry cancels them.

This procedure is more reliable than memorizing the shape of each individual formula.

Geometry and Molecular Polarity

A polar bond does not automatically make a polar molecule. Molecular polarity depends on both bond polarity and three-dimensional geometry.

In CO2, the two polar C=O bond dipoles point in opposite directions and cancel in the linear geometry, so the molecule has no net dipole moment. In H2O, the O–H bond dipoles do not cancel because the molecule is bent, so water has a net dipole moment.

Symmetry is therefore a powerful clue. Molecules with identical surrounding atoms and highly symmetric geometries, such as BF3, CH4, and SF6, can have bond dipoles that cancel. Replacing one or more surrounding atoms, or introducing lone pairs, can destroy that cancellation.

Geometry, Models, and Evidence

Chemists use several representations of three-dimensional structure. A Lewis structure emphasizes connectivity and valence electrons. Wedge-and-dash drawings indicate bonds coming out of or going behind the page. Ball-and-stick and space-filling models emphasize three-dimensional arrangement in different ways.

Molecular geometry is not determined by VSEPR alone in real research. Experimental methods such as rotational spectroscopy, vibrational spectroscopy, electron diffraction, and X-ray crystallography can provide structural information. Computational chemistry can also predict optimized structures and compare them with experimental data.

VSEPR is most successful as a qualitative model for many main-group compounds. It can be less reliable for transition-metal complexes, species with unusual electron distributions, and molecules where delocalization or weak energetic differences dominate the structure. When higher accuracy is needed, experimental evidence and quantum-chemical models take priority.

Connections to Bonding Models

You may encounter orbital hybridisation labels such as sp, sp2, and sp3 alongside geometry. These labels belong to a valence-bond model and can be useful for organizing bonding descriptions, but they are not the same thing as VSEPR. Avoid circular reasoning such as claiming that a molecule is tetrahedral because it is sp3 and then claiming it is sp3 because it is tetrahedral.

A stronger explanation begins with observable or calculable electron distribution, uses VSEPR as an approximate geometric model when appropriate, and then connects the result to other bonding models with clear assumptions.

Worked Comparisons

CO2 and H2O

CO2 and H2O are both AX2 if you count only bonded atoms, but this is not enough to determine their shapes. Carbon in CO2 has two electron domains and no lone pairs, so CO2 is linear. Oxygen in H2O has four electron domains, including two lone pairs, so its electron-domain geometry is tetrahedral and its molecular geometry is bent.

This comparison shows why lone pairs must be included in the electron-domain count.

CH4 and NH3

CH4 and NH3 each have four electron domains around the central atom. CH4 has four bonding domains and no lone pairs, so it is tetrahedral. NH3 has three bonding domains and one lone pair, so it is trigonal pyramidal. Their idealized electron-domain geometry is the same, but their molecular geometries differ.

PCl5 and SF6

PCl5 has five electron domains and is trigonal bipyramidal in the gas phase. SF6 has six electron domains and is octahedral. These examples show how increasing the domain count introduces new spatial arrangements that cannot be represented accurately by flat drawings alone.

Interactive Tasks

Quiz: Test Your Knowledge

Which molecular geometry is predicted for an AX4 species with no lone pairs on the central atom? (Tetrahedral) (!Square planar) (!Trigonal pyramidal) (!Seesaw)

How is a double bond counted in the first VSEPR electron-domain count? (As one electron domain) (!As two electron domains) (!As three electron domains) (!It is ignored)

Which statement correctly distinguishes electron-domain geometry from molecular geometry? (Electron-domain geometry includes lone pairs) (!Molecular geometry includes lone pairs as atoms) (!They are always identical) (!Electron-domain geometry ignores bonding regions)

What is the ideal bond angle in a trigonal planar arrangement? (120 degrees) (!90 degrees) (!109.5 degrees) (!180 degrees)

Which molecular shape is expected for AX3E? (Trigonal pyramidal) (!Tetrahedral) (!Linear) (!Square planar)

Why is the H–O–H angle in water smaller than the ideal tetrahedral angle? (Lone pairs repel bonding domains strongly) (!Oxygen has no lone pairs) (!Water is trigonal planar) (!Hydrogen forms four bonds)

Which position is preferred by a lone pair in a trigonal bipyramidal electron-domain geometry? (Equatorial) (!Axial) (!Linear) (!Octahedral)

Which molecular geometry is associated with AX4E2? (Square planar) (!Square pyramidal) (!Trigonal bipyramidal) (!Tetrahedral)

Why is carbon dioxide nonpolar despite having polar carbon oxygen bonds? (The bond dipoles cancel in a linear geometry) (!The carbon oxygen bonds are ionic) (!Carbon dioxide has a tetrahedral shape) (!Oxygen and carbon have equal electronegativity)

Which method can provide experimental information about molecular geometry? (X-ray crystallography) (!Alphabetical sorting) (!Stoichiometric naming) (!Balancing coefficients alone)

Memory Game

Linear	Electron-domain arrangement with two regions pointing in opposite directions
Tetrahedral	Four-domain arrangement with an ideal angle near 109.5 degrees
Trigonal pyramidal	Molecular shape produced by three bonds and one lone pair
Seesaw	Molecular shape produced by four bonds and one lone pair in a five-domain arrangement
Octahedral	Six-domain arrangement with adjacent positions 90 degrees apart
Equatorial	Less crowded position preferred by lone pairs in a trigonal bipyramid
Dipole	Vector quantity used when reasoning about molecular polarity

Drag and Drop

Match the correct terms.	Topic
Tetrahedral	Methane central carbon
Bent	Water central oxygen
Trigonal planar	Boron trifluoride central boron
Trigonal bipyramidal	Phosphorus pentachloride central phosphorus
Octahedral	Sulfur hexafluoride central sulfur

...

Crossword Puzzle

Tetrahedral	Which geometry has four electron domains directed toward the corners of a tetrahedron?
Linear	Which geometry places two bonding directions 180 degrees apart?
Equatorial	What type of position is preferred by a lone pair in a trigonal bipyramid?
Octahedral	Which six-domain geometry has 90 degree angles between adjacent positions?
Pyramidal	Which word completes the molecular shape name trigonal what for ammonia?
Polarity	What molecular property depends on the vector sum of bond dipoles?

LearningApps

Cloze Text

Open-Ended Tasks

Easy

Molecule model gallery: Build or sketch models of CO2, BF3, CH4, NH3, and H2O, label each molecular geometry, and photograph or scan your results.
Wedge and dash drawing: Convert three flat Lewis structures into wedge-and-dash drawings and explain what each wedge and dash means in three-dimensional space.
Shape comparison card: Create a one-page visual comparison of linear, trigonal planar, and tetrahedral geometries with one example and ideal bond angle for each.
Polarity prediction: Predict whether CO2, H2O, BF3, and NH3 are polar or nonpolar, then justify each prediction using geometry rather than bond polarity alone.

Standard

VSEPR explainer video: Produce a three-minute video that teaches a learner how to move from a Lewis structure to molecular geometry using at least three examples.
Molecular geometry interview: Interview a chemistry teacher, laboratory technician, pharmacist, or materials scientist about one situation in which molecular shape matters, then summarize the connection to course concepts.
Bond angle investigation: Compare ideal VSEPR angles with reported or modelled angles for CH4, NH3, and H2O, and explain the trend using lone-pair repulsion.
Five-domain model: Construct a trigonal bipyramidal model, mark axial and equatorial sites, then demonstrate why lone pairs prefer equatorial positions.

Advanced

Geometry and reactivity study: Choose two molecules with related formulas but different geometries and investigate how their shapes influence a chemical or physical property.
Model limitations analysis: Find a molecule or complex for which simple VSEPR gives an incomplete description, and write a critical explanation of what additional evidence or theory is needed.
Spectroscopy and structure report: Research how one experimental technique can reveal molecular geometry and create a short report connecting the measured signal to structural information.
Computational geometry project: Use an appropriate molecular modelling tool to optimize several small molecules, compare calculated bond angles with VSEPR predictions, and discuss agreements and deviations.

Learning Assessment

Reasoning from Lewis structures: Given unfamiliar main-group molecules and ions, determine electron-domain and molecular geometries and justify every step from the Lewis structures.
Comparing models: Explain why a Lewis structure, a VSEPR model, and a three-dimensional molecular model communicate different information about the same substance.
Polarity transfer task: Predict the polarity of several molecules with polar bonds and defend each conclusion using symmetry and vector cancellation.
Error analysis: Diagnose a worked solution that confuses electron-domain geometry with molecular geometry and rewrite the explanation correctly.
Evidence-based structure: Compare a VSEPR prediction with an experimentally measured bond angle and explain why ideal and measured values may differ.
Novel application: Apply the VSEPR workflow to a molecule not used in the course examples and communicate the result with a labelled three-dimensional representation.

Evidence of Learning

Strong evidence of learning includes accurate Lewis structures; correct electron-domain counts; correct use of AXE notation; justified predictions of electron-domain and molecular geometry; meaningful estimates of bond angles; explanations of lone-pair effects; polarity predictions based on vector cancellation; clear three-dimensional drawings or physical models; comparisons between ideal and measured structures; and reasoned discussion of where VSEPR is useful or limited.

Products such as model photographs, annotated diagrams, short explanatory videos, investigation reports, interview summaries, computational comparisons, and written error analyses can demonstrate both conceptual knowledge and practical transfer. High-quality work should make assumptions explicit and distinguish a model prediction from experimental evidence.