Science
Lewis structures: molecules, shapes and forces
Draw simple Lewis structures, connect electron pairs with molecular shape and compare the named interactions between molecules. Two pupil pages include water and carbon-dioxide models, a shape key, methane and ammonia tasks, and a written explanation. The companion provides electron counts, angles and the limits of the force matching.
Compare the finished Lewis models and molecular shapes. These are simplified diagrams with their source angle labels.
Read the complete materialRead the material
Draw simple Lewis structures, connect electron pairs with molecular shape and compare the named interactions between molecules. Two pupil pages include water and carbon-dioxide models, a shape key, methane and ammonia tasks, and a written explanation. The companion provides electron counts, angles and the limits of the force matching.
Pupil worksheet. Use the preview for diagrams and layout.
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CHEMISTRY AGES 14 TO 16 BONDING Lewis structures Draw the bonds, add the lone pairs, name the shape. NAME CLASS DATE LEARNING GOAL Draw Lewis structures of small molecules, name their shapes and say which forces act between molecules. SUCCESS CRITERIA I can add up valence electrons. I can draw bonds and lone pairs so every atom is full. I can name a shape and a force. E N T RY 1 Draw a Lewis structure NOTES TO SELF VALENCE ELECTRONS Each atom brings its own number. READING IT A line is a bond: two shared electrons. A pair of dots is a lone pair. Add up the valence electrons of all the atoms. 1 Join the atoms with single bonds. Draw each bond as a line. 2 Give each atom its lone pairs, drawn as two dots, until it is full. 3 Count. Every atom is full (hydrogen 2, others 8) and the total is right. If an atom is short, share a lone pair as a second bond. 4 H 1 C 4 N 5 O 6 Cl 7 E N T RY 2 Two finished models Study them ELECTRONS 2 bonds + 2 lone pairs Shape: bent Bond angle: about 105° ELECTRONS 2 double bonds + 4 lone pairs Shape: linear Bond angle: 180° H2O O H H 8 CO2 C O O 16 E N T RY 3 Shapes key Use it on page 2 Bond angle 180° Bond angle about 105° Bond angle about 107° Bond angle about 109.5° LINEAR BENT TRIGONAL PYRAMIDAL TETRAHEDRAL Pupil activity | Page 1 of 2
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CHEMISTRY AGES 14 TO 16 YOUR TURN Shapes and forces NAME CLASS DATE E N T RY 1 Draw CH4 Atoms: 1 C, 4 H Total valence electrons Shape Bond angle: about ° E N T RY 2 Draw NH3 Atoms: 1 N, 3 H Total valence electrons Shape Bond angle: about ° DRAW HERE DRAW HERE E N T RY 3 Match the force MOLECULE A H2O 1 London (dispersion) forces B CH4 NH3 C 2 hydrogen bonds D CO2 E N T RY 4 Explain 3 marks Why is water a liquid at room temperature but methane a gas? Use: HYDROGEN BONDS STRONGER MORE ENERGY Strongest named interaction In pure condensed samples; all also have dispersion. Pupil activity | Page 2 of 2
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Teacher companion Lewis structures: molecules, shapes and forces Structures and shapes H2O: 8 valence electrons, two O–H bonds, two lone pairs on O; bent, about 104.5° (105° accepted). CO2: 16 valence electrons, O=C=O, two lone pairs on each O; linear 180°. CH4: 8 valence electrons, four C–H single bonds, no lone pair on C; tetrahedral about109.5°. NH3: 8 valence electrons, three N–H bonds, one lone pair on N; trigonal pyramidal about107°. Forces matching For the strongest named interaction among the two choices: H2O and NH3, hydrogen bonding; CH4 and CO2, London dispersion. All four also have London dispersion. Water and ammonia have other dipolar interactions too. The diagram must not be read as mutually exclusive presence/absence. Explanation, 3 marks Water molecules can form hydrogen bonds (1), these are stronger than the dispersion interactions between small methane molecules (1), so more energy is needed to separate water molecules into a gas under comparable conditions (1). Specify ordinary room temperature and pressure; do not compare different substances at arbitrary pressures. Model scope and source The electron-count method applies to the stated simple neutral examples; it is not a universal octet rule. https://www.chem.purdue.edu/gchelp/liquids/imf.html Teacher companion | Page 1 of 1
A teaching sequence
Pupils should account for eight valence electrons in methane and ammonia, draw bonds and lone pairs, and identify their shapes. They should explain the water-methane comparison in terms of interactions between molecules, while recognising that all four examples also have dispersion forces.
Compare the finished Lewis models and molecular shapes. These are simplified diagrams with their source angle labels.
Read the complete materialCount and allocate electrons
Read the drawing method and check both the total electron count and each atom’s allocation. Ask pupils what a bond line represents before they draw their own structures.
What does one bond line represent in the supplied model?
Draw methane and ammonia
Draw methane and ammonia and compare the central atom’s bonds and lone pairs. Use the shape key to name tetrahedral and trigonal pyramidal shapes with their approximate angles.
How does the central atom's lone-pair count affect the shape?
Choose the strongest named interaction
Match each molecule to the strongest named interaction among the two printed choices. Discuss why choosing hydrogen bonding does not mean dispersion forces are absent.
Why does naming hydrogen bonding not remove dispersion forces?
Explain water and methane
Write the water-methane explanation under ordinary room conditions. Ask whether the answer concerns separating molecules or breaking the covalent bonds within them, then refine unclear wording.
Are you discussing separation between molecules or bonds within them?
Before you teach
Look for the thinking behind an answer, choose the support your class needs and leave room for an independent response.
What to look for
Look for four C-H bonds and no central lone pair in methane, and three N-H bonds plus one lone pair in ammonia. A useful explanation says more energy is needed to separate water molecules because of hydrogen bonding. If a pupil removes all dispersion forces from water, revisit the matching instruction’s stated scope.
Give a little support
Use a running electron tally beside the structure: total available, electrons in bonds and electrons in lone pairs. Recheck hydrogen’s two-electron allocation before adding dots elsewhere. Compare the two worked models with the shape key before attempting an independent shape-and-angle answer.
Take the idea further
Revisit the two worked molecules and explain why both contain double or single bonds within the molecule but receive different answers in the interaction task. Refine one explanation to distinguish bonding within molecules from forces between them, without extending the simple method to unprovided examples.
Things to keep in mind
The simple neutral examples use a scoped electron-count model. Compare pure samples under ordinary room-temperature and pressure conditions.
Keep the idea. Make it yours.
Bring your topic, learning goal and class context to PLAI™, then review what you create.
Ideas to try with your class
Sample teaching perspectives on what to try, what to adapt and what to watch for.
Meet The Creators
Meet a creator whose teaching interests connect with this template.
Reread Reminder
The two-page chemistry activity moves from finished water and carbon dioxide models to methane and ammonia, asking for valence-electron totals, bonds, lone pairs, shape and angle. A final comparison concerns water and methane intermolecular forces.
Bright Idea Break
For ages 14 to 16, the structures key distinguishes bent, linear, trigonal pyramidal and tetrahedral shapes. Pupils have to account for eight electrons around non-hydrogen atoms before naming a geometry.
Quietly Curious
Use the water and carbon dioxide diagrams to model how lines and lone pairs represent electrons, then build CH₄ and NH₃ separately. Keep the valence-electron table visible during the count.
Tea at Breaktime
The final comparison should retain the material’s caution that all four examples have dispersion forces. Check each molecule’s shape and intermolecular-force explanation against the taught chemistry model; the drawing is not a three-dimensional model.