Molecular Geometry & Polarity · Grade 11 Science
Chapter: Bonding, Formulas & Molecular StructureMolecular Geometry & Polarity
Learning goals
- I can use VSEPR theory to predict the shape of a simple molecule from its bonding groups and lone pairs.
- I can decide whether a bond and a whole molecule are polar or nonpolar, and name the main intermolecular forces.
Readings for this unit
Before this unit
Lewis structures, domain counting, and vector cancellation.
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Start with something to explain
Can a molecule have polar bonds but no permanent net dipole?
Illustrative evidence case. The selected comparison pairs linear CO₂ with bent H₂O.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Can a molecule have polar bonds but no permanent net dipole?
Your first model
Draw directional bond contributions and add them using the geometry.
Build the language from the idea
- VSEPR theory
- Valence Shell Electron Pair Repulsion — the idea that electron groups around a central atom spread out as far apart as possible, which sets the molecule's shape.
- molecular geometry
- The three-dimensional shape formed by the atoms in a molecule.
- electronegativity
- A measure of how strongly an atom pulls on the shared electrons in a bond.
- polar bond
- A covalent bond in which electrons are shared unequally, creating a slightly positive end and a slightly negative end.
- nonpolar bond
- A covalent bond in which electrons are shared equally, with no separation of charge.
- dipole
- A separation of partial positive and partial negative charge across a bond or a whole molecule.
- intermolecular force
- An attraction between separate molecules; these are much weaker than the bonds within a molecule.
- hydrogen bond
- A strong intermolecular attraction between a hydrogen bonded to N, O, or F and a lone pair on the N, O, or F of a neighboring molecule.
A Lewis structure drawn flat on paper hides one of the most important facts about a molecule: it is a real, three-dimensional object with a definite shape. And shape is not a minor detail — it decides how a molecule packs, dissolves, smells, and reacts, and whether a life-saving drug fits its target or misses entirely. The tool chemists use to predict that shape from a simple Lewis structure is VSEPR theory, which stands for Valence Shell Electron Pair Repulsion — a long name for one commonsense idea.
The idea is this: the groups of electrons around a central atom all carry negative charge, so they repel one another and spread out as *far apart as possible*, and that spreading fixes the molecule's shape. Count the 'electron groups' on the central atom — each single, double, or triple bond counts as one group, and so does each lone pair. Two groups push to opposite sides and give a linear shape (180°), as in carbon dioxide, O=C=O. Three groups spread into a flat triangle, or trigonal planar (120°). Four groups point to the corners of a tetrahedron (about 109.5°), the shape of methane, CH₄.
Lone pairs are the plot twist. They take up space and repel just like bonding groups — actually a little *more* strongly — but you don't 'see' them when you name the shape, because the name describes only where the *atoms* are. The oxygen in water has four electron groups (two bonds plus two lone pairs), so they arrange tetrahedrally, but with only two hydrogens attached the visible molecule is bent, its H–O–H angle squeezed to about 104.5°. The nitrogen in ammonia has three bonds and one lone pair, giving a trigonal pyramidal shape. The invisible lone pairs quietly push the atoms into these bent and pyramidal forms.
Shape becomes truly powerful when you combine it with polarity. Recall that in a covalent bond atoms share electrons — but they don't always share them *equally*. Electronegativity measures how strongly an atom pulls on that shared pair, and when two bonded atoms differ in electronegativity, the greedier one hogs the electrons, taking on a slight negative charge while its partner is left slightly positive. That lopsided bond is a polar bond, and the little separation of charge is a dipole. When the two atoms are identical, as in O₂, the sharing is perfectly even and the bond is nonpolar.
Here's the subtle part that trips up many students: a molecule can be full of polar bonds and yet be nonpolar *overall* — it all depends on the shape. Think of each polar bond as an arrow (a dipole) pointing toward the greedier atom. If the molecule is symmetric, those arrows point in balanced directions and cancel out, leaving no net pull. Carbon dioxide is the perfect example: its two C=O bonds are polar, but the molecule is linear, so the two arrows point in exactly opposite directions and cancel — CO₂ is nonpolar. Water has equally polar O–H bonds, but because it is *bent*, the arrows don't cancel; they add up to a net dipole, so water is polar. Same kind of bonds, opposite result — all because of shape.
Molecular polarity, in turn, controls the intermolecular forces — the attractions *between* whole molecules that decide whether a substance is a gas, a liquid, or a solid. The weakest, present in everything, are fleeting London dispersion forces; polar molecules feel stronger dipole–dipole attractions; and the strongest of all is the hydrogen bond, a powerful attraction between a hydrogen bonded to nitrogen, oxygen, or fluorine and a lone pair on a neighboring molecule. Water's abundant hydrogen bonds are why it boils at a startlingly high temperature for such a small molecule, why ice floats, and why it dissolves so much. A simple rule falls out of all this — 'like dissolves like': polar dissolves polar and nonpolar dissolves nonpolar, which is exactly why oil and water refuse to mix.
Model check: Polar bonds do not automatically make a polar molecule. CO₂ is linear with equal opposite bond dipoles that cancel; bent water has a nonzero molecular dipole.
Build your understanding
Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.
Part 1 · Count electron domains around a center
The VSEPR model uses repulsions among valence-electron regions to predict common molecular geometries. An electron domain is a bonding direction or lone-pair region around the chosen central atom; a double or triple bond counts as one direction in this model. CO₂ has two central bonding domains and no central lone pair, so the ideal geometry is linear. VSEPR is a model with a domain of use, not a complete quantum calculation or a rule that every atom in any molecule must be its center.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
How many central domains does O=C=O have, even though it contains four shared bonding pairs?
Visual reasoning — after your attempt
Two domains around carbon. Each double bond occupies one bonding direction in VSEPR counting.
In the introductory VSEPR model, bonding regions and lone pairs arrange to reduce repulsion. A single, double, or triple bond counts as one domain for basic geometry prediction. Electron-domain geometry includes lone pairs; molecular geometry names the arrangement of atom positions. These are related but distinct descriptions.
A center with four domains has tetrahedral electron-domain geometry. With two bonded atoms and two lone pairs, its molecular geometry is bent, not tetrahedral.
Pause and explain
Why do water’s four domains not mean four surrounding atoms?
Check your reasoning after answering
Two domains are lone pairs and two are O–H bonding regions. Molecular geometry describes the atom positions, which form a bent shape.
Part 2 · Use lone pairs to refine geometry
A lone pair contributes an electron domain but is not an atom in the molecule’s visible nuclear framework. NH₃ has four central domains: three N–H bonding directions and one lone-pair region. Its electron-domain arrangement is approximately tetrahedral, while the molecular shape is trigonal pyramidal. Wedge-and-dash drawings project depth onto a page; they are not flat measured angles. Lone-pair interactions often change bond angles from idealized domain values, so distinguish a model prediction from a precise measured geometry.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why are tetrahedral and trigonal pyramidal both relevant to NH₃ without being synonyms?
Visual reasoning — after your attempt
Tetrahedral describes all four electron domains; trigonal pyramidal describes the positions of the three bonded atoms relative to nitrogen, excluding the lone pair.
Lone pairs influence bonding-region positions, and observed angles may differ from ideal domain angles. VSEPR is a useful qualitative model, not a universal precise-angle calculator. Identify total electrons and a reasonable Lewis structure before naming a geometry.
NH₃ has three bonds and one lone pair around nitrogen, giving trigonal-pyramidal molecular geometry. CH₄ has four bonds with no lone pairs and tetrahedral geometry.
Optional guided lab · Change lone pairs and compare two kinds of geometry
Optional guided lab · Change lone pairs and compare two kinds of geometry
Optional external exploration — WeduLab. Complete the local explanation first. The task and practice record here are independently written for this course. Choose this in place of comparable practice or allow extra time; it does not add a required session to the year plan.
Predict the atom arrangement for AX4, AX3E and AX2E2. Here A is the central atom, X counts bonded atoms and E counts lone pairs on that center. An electron domain is one region of electron density; a single, double or triple bond counts as one domain in this VSEPR count. These three selected cases all have four domains.
Explore for about 15–20 minutes after the lone-pair explanation. In the VSEPR sandbox select AX4, AX3E and AX2E2 in that order. For each, sketch all four domains, circle only the atom positions and record the displayed angle as a sandbox result. Rotate the view so a three-dimensional shape is not mistaken for a flat drawing. Compare electron geometry with molecular geometry; use the supplied table if the controls are inaccessible.
This is a repulsion-based sandbox. Its live angle is a model output, not a calibrated measurement of a particular molecule. The ideal tetrahedral angle is about 109.5°; real gaseous water has an H–O–H angle about 104.5°. Do not relabel an arbitrary AX2E2 readout as measured water. Colored lobes are representations of electron density, not solid objects or electron orbits. Advanced hybridization, expanded-octet and chirality topics are optional extensions beyond this task.
No-lab route: the original supplied record below contains everything needed for the question. It is not measured evidence collected from WeduLab. Label any separate observations you collect as simulation output, with settings and date.
- Open WeduLab · MoleCraft VSEPR lab (external, English)
- OpenStax Chemistry 2e · molecular structure and polarity reference
| Original four-domain comparison | Bonded atoms / lone pairs | Electron-domain geometry | Molecular geometry |
|---|---|---|---|
| AX4 | 4 / 0 | Tetrahedral | Tetrahedral |
| AX3E | 3 / 1 | Tetrahedral | Trigonal pyramidal |
| AX2E2 | 2 / 2 | Tetrahedral | Bent |
Question 1
Why can all three rows have the same electron-domain geometry but different molecular shapes? A sandbox shows an AX2E2 angle near 108°. Can you report that as the measured bond angle of water? Explain, then use bond-dipole directions to compare the cancellation in ideal CO2 and H2O models.
Worked solution and marking guidance
Electron geometry counts all four regions, while molecular geometry describes the positions of bonded atoms and excludes lone-pair positions.
A sandbox calculates a simplified configuration. Its value does not establish a measured molecular angle; water is about 104.5° under the stated reference conditions.
In linear CO2 the equal C–O bond dipoles oppose and cancel. In bent H2O, the O–H bond dipoles do not cancel, so the molecule is polar.
Four domains do not always mean a tetrahedral molecular shape. The numerical sandbox angle must be labeled as model output. CO2 can have polar bonds but no net molecular dipole, whereas bent water has a net dipole.
- Distinguish domains, bonded atoms and lone pairs.
- Label a model output separately from a measured molecular property.
- Use shape and vector cancellation when explaining polarity.
Pause and explain
Why are NH₃ and CH₄ not assigned the same molecular geometry?
Check your reasoning after answering
Both have four domains, but ammonia has a lone pair instead of a fourth bonded atom. Molecular geometry counts atom positions, not all domains.
Part 3 · Combine bond dipoles as vectors
A dipole moment has direction as well as magnitude. Symmetry can make equal bond contributions cancel, as in an isolated ideal linear CO₂ molecule. In bent H₂O, the corresponding contributions do not cancel, producing a net molecular dipole. A bond can therefore be polar while its entire molecule has no permanent net dipole. The selected arrows point toward the negative ends as a classroom convention; a quantitative dipole calculation needs the actual charge distribution rather than these unscaled arrows alone.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why do two polar C–O bonds not automatically make CO₂ a polar molecule?
Visual reasoning — after your attempt
In the symmetric linear molecule, equal opposing contributions cancel. Geometry is needed along with bond polarity.
A bond dipole points toward the more electronegative side in the common arrow convention. Molecular polarity depends on geometry and whether dipoles cancel. Symmetry must include the identity of surrounding groups; a symmetric-looking center with different substituents may not cancel.
Linear CO₂ has equal opposite bond dipoles. Tetrahedral CH₄ is symmetric, while CH₃Cl has a distinct substituent and a net dipole. Draw arrows instead of deciding from one bond alone.
Pause and explain
Why can replacing one H in CH₄ with Cl change molecular polarity?
Check your reasoning after answering
It breaks the equal-substituent symmetry and changes the dipole sum. Geometry alone without substituent identity does not settle cancellation.
Part 4 · Connect polarity to interactions without a single-factor rule
Molecular polarity affects interactions with neighboring molecules or ions, but bulk behavior also depends on molecular size, shape, polarizability, phase and conditions. Polarizability describes how readily electron density can be distorted. Two substances with different boiling points cannot be explained by counting polar bonds alone. In solutions, disruption of existing interactions and formation of new ones both matter. Use a matched comparison and identify the relevant mechanisms rather than treating polar as a universal synonym for high boiling or water-soluble.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why might two molecules with the same formula have different boiling behavior?
Visual reasoning — after your attempt
Their shapes and intermolecular contact arrangements can differ, affecting dispersion and other interactions even when composition is the same.
Molecular polarity can affect attractions and solubility, but dispersion forces, size, shape, and hydrogen bonding also matter. Like dissolves like is a useful tendency, not a complete calculation of solubility. A nonpolar large molecule may have a higher boiling point than a small polar one.
Compare molecules using complete interaction evidence rather than only whether a dipole exists. State the solvent, temperature, and measured property when testing a polarity-based prediction.
Pause and explain
Does polar automatically mean higher boiling than every nonpolar substance?
Check your reasoning after answering
No. Molecular size, polarizability, shape, and other interactions matter. Polarity is one contributor, not a universal ranking rule.
Apply the whole unit · Predict three molecular shapes
Illustrative supplied Lewis patterns: CO₂ has two bonding domains and no lone pairs on C; H₂O has two bonds and two lone pairs on O; NH₃ has three bonds and one lone pair on N.
Your task
Name electron-domain and molecular geometries, draw dipole directions, and predict whole-molecule polarity. Explain why the prediction does not alone provide exact boiling points.
What your response should show
- Choose the appropriate chemical model or relationship and state its conditions.
- Show substitutions, units, particle or charge counts, and intermediate reasoning as relevant.
- Use supplied evidence to justify the conclusion; distinguish observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
CO₂: two domains, linear domain and molecular geometry.
H₂O: four domains, tetrahedral domain arrangement, bent molecular geometry.
NH₃: four domains, tetrahedral domain arrangement, trigonal-pyramidal molecular geometry.
CO₂ dipoles cancel; bent water and pyramidal ammonia have net dipoles in these models.
The supplied models predict nonpolar CO₂ and polar H₂O/NH₃. Lone pairs belong in the domain count even though molecular shape names describe atom positions. Exact boiling points need all intermolecular interactions and suitable reference data; shape and polarity do not constitute complete property measurements.
Scientific references and source-model foundations
Parent / teacher teaching notes
Have students make both a domain table and a molecular sketch. Do not require precise bond angles from VSEPR alone. Check dipole cancellation rather than memorized polar/nonpolar labels. Ask the learner to explain each particle symbol, axis, unit and arrow before calculating. Distinguish hypothetical models and calculated curves from measured records. Public OpenSciEd excerpts here are selected foundation prompts, not complete high-school teacher editions or an endorsement. All required advanced explanations and practice are provided locally.
The key idea
VSEPR theory says electron groups spread out to set a molecule's shape, and a molecule's overall polarity depends on both its bond polarities and whether that shape lets the dipoles cancel — which in turn sets the intermolecular forces that shape a substance's properties.
Electron Groups on the Central Atom (VSEPR)
electron groups = (atoms bonded to the central atom) + (lone pairs on the central atom)
each single, double, or triple bond counts as ONE group. The number of groups sets the shape: 2 = linear, 3 = trigonal planar, 4 = tetrahedral. Example: the O in H₂O has 2 bonded atoms + 2 lone pairs = 4 groups.
Worked examples
Example 1
- Identify the central atom and count its electron groups: carbon is bonded to 4 hydrogens and has no lone pairs, giving 4 electron groups.
- Four groups repel and spread as far apart as possible, pointing toward the corners of a tetrahedron.
- With an atom at each corner and none missing, the molecule is tetrahedral, with bond angles of about 109.5°.
Example 2 · Worked application
- Each bond has a dipole.
- In the symmetric linear geometry, equal opposing dipoles cancel.
- Molecular polarity is their vector sum, not just the presence of polar bonds.
Compare CO₂ with bent H₂O, where dipoles do not cancel.
Example 3
- Use VSEPR: the central carbon has 2 bonding groups (the two double bonds) and no lone pairs, so the molecule is linear (O=C=O at 180°).
- Each C=O bond is polar because oxygen pulls the shared electrons more strongly, creating two dipoles that point outward in exactly opposite directions.
- Because the molecule is symmetric and linear, the two equal dipoles cancel each other out, leaving no net dipole.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: Compare CO₂ with bent H₂O, where dipoles do not cancel.
- Symmetry can cancel bond dipoles; geometry and charge distribution determine the net dipole.
Compare CO₂ with bent H₂O, where dipoles do not cancel.
Example 5 · Reason from the visual model
- A dipole moment has direction as well as magnitude. Symmetry can make equal bond contributions cancel, as in an isolated ideal linear CO₂ molecule. In bent H₂O, the corresponding contributions do not cancel, producing a net molecular dipole. A bond can therefore be polar while its entire molecule has no permanent net dipole. The selected arrows point toward the negative ends as a classroom convention; a quantitative dipole calculation needs the actual charge distribution rather than these unscaled arrows alone.
- In the symmetric linear molecule, equal opposing contributions cancel. Geometry is needed along with bond polarity.
Original qualitative vector comparison. Connectors show bond directions and are not complete Lewis structures; water angles are schematic here.
Avoid these mistakes
- Polar bonds do not automatically make a polar molecule.
Check your understanding
Compare CO₂ and water polarity.
Discuss the answer
CO₂ is linear with equal opposite bond dipoles that cancel; bent water has a nonzero molecular dipole.
Return to the phenomenon
Return to the initial case. Can a molecule have polar bonds but no permanent net dipole? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Equal opposing contributions cancel in ideal linear CO₂; bent water retains a nonzero net contribution. Bond polarity and molecular polarity answer different questions.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, calculate and revise
Keep a first model and an evidence-supported revision. Attempt each task before opening guidance; all required explanations and supplied-data routes are available locally.
Activity 1 · Return to the opening case
Use the opening phenomenon, unit definitions and explained Parts 1–4.
Your task
Return to the initial case. Can a molecule have polar bonds but no permanent net dipole? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use the stated observations and appropriate units.
- Connect a labeled model with the chemical mechanism.
- State assumptions and an evidence limitation.
- Explain a substantive revision or next test.
Hint
Keep species identity, material transfer and energy account distinct.
Response guidance — discuss after your attempt
Equal opposing contributions cancel in ideal linear CO₂; bent water retains a nonzero net contribution. Bond polarity and molecular polarity answer different questions.
Activity 2 · Transfer to a changed case
Cover the worked answer and use the explained models, relationships and notebook.
Your task
A molecule has four electron domains around its center, with two bonding pairs and two lone pairs. Predict electron-domain geometry and molecular shape; explain why bond angles need not be 109.5°.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the chemical mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name known quantities and conditions, show intermediate steps and check conservation.
Response guidance — discuss after your attempt
The electron-domain arrangement is approximately tetrahedral and the molecular shape is bent. Lone-pair interactions and the actual chemical species alter angles; 109.5° is an ideal tetrahedral reference rather than an exact universal bond angle.
Words to know
- VSEPR theory
- Valence Shell Electron Pair Repulsion — the idea that electron groups around a central atom spread out as far apart as possible, which sets the molecule's shape.
- molecular geometry
- The three-dimensional shape formed by the atoms in a molecule.
- electronegativity
- A measure of how strongly an atom pulls on the shared electrons in a bond.
- polar bond
- A covalent bond in which electrons are shared unequally, creating a slightly positive end and a slightly negative end.
- nonpolar bond
- A covalent bond in which electrons are shared equally, with no separation of charge.
- dipole
- A separation of partial positive and partial negative charge across a bond or a whole molecule.
- intermolecular force
- An attraction between separate molecules; these are much weaker than the bonds within a molecule.
- hydrogen bond
- A strong intermolecular attraction between a hydrogen bonded to N, O, or F and a lone pair on the N, O, or F of a neighboring molecule.
The two Ronalds who named the shapes
The tidy rule that electron groups spread out to set a molecule's shape has a name and an origin: VSEPR theory, developed in 1957 by Ronald Gillespie and Ronald Nyholm. Their insight was that you don't need heavy quantum calculations to predict a molecule's geometry — you just count the electron groups around the central atom and let their mutual repulsion do the rest. It remains one of chemistry's best examples of a simple idea with enormous predictive power.
How lone pairs squeeze the angles
Lone pairs don't just bend a molecule — they push harder than bonding pairs, because a lone pair is held by only one nucleus and so spreads out more. That extra shove shows up in the numbers: methane (no lone pairs) sits at a roomy 109.5°, ammonia (one lone pair) is squeezed to about 107°, and water (two lone pairs) is pinched all the way down to 104.5°. The order of pushiness runs lone-pair–lone-pair > lone-pair–bond > bond–bond.
The bent molecule that lets fish survive winter
Water's humble bent shape has outsized consequences. Because the molecule is bent and polar, its many hydrogen bonds lock solid water into an open, roomy lattice — so ice is less dense than liquid water and floats. A freezing pond therefore forms an insulating lid of ice on top while liquid water survives beneath it, letting fish live through the winter. Almost every other substance sinks when it freezes; water's shape makes it a life-preserving exception.
Why soap can wash away grease
'Like dissolves like' says polar mixes with polar and nonpolar with nonpolar — so how does water ever wash off greasy oil? The secret is soap, whose long molecules have a polar head that loves water and a nonpolar tail that clings to grease. The tails bury themselves in the oil while the heads face the water, breaking the grease into tiny droplets that rinse away. A single cleverly shaped molecule bridges two worlds that normally refuse to mix.
Shape, polarity, and the warming planet
A molecule can trap heat only if its shape lets its charges shift as it vibrates. Symmetric O₂ and N₂ — the bulk of the air — stay perfectly balanced when they wiggle, so they let Earth's infrared heat pass straight by. But bent water, and even linear carbon dioxide when it bends and flexes, develop a changing dipole that absorbs that infrared and radiates it back down. That difference in molecular shape is precisely why CO₂ and water vapor are greenhouse gases while the main gases of the air are not.
In the real world
A molecule's shape and polarity decide how it behaves. Water's bent, polar shape lets it dissolve salts and sugars and gives it the surface tension that lets insects walk on a pond, while nonpolar oils won't mix with it — 'like dissolves like.' Drug designers obsess over molecular shape because a medicine works only if it fits the exact target in your body.
Exit check
Compare CO₂ and water polarity.
Check your explanation after answering
CO₂ is linear with equal opposite bond dipoles that cancel; bent water has a nonzero molecular dipole.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.