Chemistry 11–12 · Year 11

Predicting and building molecular shapes with Lewis diagrams, VSEPR and model kits

Module 1: Properties and Structure of Matter

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Electron domains around a central atom spread as far apart as possible, so counting bonding pairs and lone pairs predicts a molecule's shape, and the shape with the bond polarities decides whether the molecule is polar.

What you need

Molecular model kits (ball-and-stick with tetrahedral, trigonal and linear centres and lone-pair pieces), one per pair
Protractor
Periodic table with electronegativity values
Worksheet list: H2, HCl, CO2, HCN, BF3, C2H4, CH4, NH3, H2O, CH2Cl2

How to do it

Draw the Lewis dot diagram of each molecule and count the electron domains (bonds and lone pairs) on the central atom; treat a double or triple bond as one domain.
Predict the electron-domain geometry and the molecular shape, and the ideal bond angle.
Build each molecule with the kit, including lone-pair pieces where the kit has them; measure one bond angle on the model with the protractor.
Mark each bond as polar or non-polar from the electronegativity difference, then decide from the model's symmetry whether the bond dipoles cancel.
Check each prediction in the simulation and resolve any disagreement.
Classify the molecules as polar or non-polar and give the reason for each from its shape and bond polarities.

What you should see

CO2 and HCN are linear (180 degrees), BF3 trigonal planar (120 degrees), C2H4 planar with about 120 degrees at each carbon, CH4 and CH2Cl2 tetrahedral (ideal 109.5 degrees, the arccos of minus one third), NH3 trigonal pyramidal and H2O bent, both with angles below 109.5 degrees because lone pairs repel more strongly than bonding pairs. CO2, BF3, CH4, C2H4 and H2 are non-polar (symmetric shapes cancel their bond dipoles, or the bonds are non-polar); HCl, HCN, NH3, H2O and CH2Cl2 are polar.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Molecules are flat, as drawn on paper (four domains point to the corners of a tetrahedron).
A molecule with polar bonds must be polar (symmetric molecules such as CO2 and CH4 cancel their bond dipoles).
Lone pairs take up no space (they repel more strongly than bonding pairs and close the bond angle in NH3 and H2O).

Safety card

Low riskLearners carry it out

Hazards

small model pieces

Controls

kits counted back in at the end

Note

No chemicals or heat are used, so the NSW Department of Education Chemical Safety in Schools package does not apply; ordinary classroom supervision.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-8 CH11/12-4 CH11/12-7
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-01 CH-11WS-04 CH-11WS-07
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities