CHEM20 Session 07 — VSEPR shapes

Delivery id: CHEM20 (SCN2796)

Time: 75 minutes

Unit A: Diversity of Matter and Chemical Bonding

Alberta POS: https://open.alberta.ca/publications/3950147-updated-2014

Official outcomes

Code	Type	Role today
20-A2.5k	Knowledge	Apply VSEPR to predict linear, angular (V-shaped, bent), tetrahedral, trigonal pyramidal, trigonal planar
20-A2.6k	Knowledge	Illustrate by drawing or building models of simple molecular substances
20-A2.4k	Knowledge	Lewis structures as the input to VSEPR

Learning targets

1. State VSEPR: valence-shell electron pairs (bonding and lone) repel and occupy positions that maximize separation.
2. Assign AXE notation (A = central, X = bonded atoms, E = lone pairs on central atom) and the five Chem 20 shapes.
3. Predict shapes for CO2, BeCl2 (linear), BF3 or H2CO (trigonal planar), CH4 / CCl4 (tetrahedral), NH3 (trigonal pyramidal), H2O / SO2 (bent), HCN (linear).
4. Build or draw 3-D representations; a 2-D Lewis diagram is not the molecular shape.

Prerequisites

Session 06 Lewis structures. Lone pairs on the central atom matter; lone pairs on terminal atoms do not change AXE.

Diagnostic/retrieval

1. Lewis of H2O: how many lone pairs on O?
2. Lewis of CO2: how many lone pairs on C?
3. Is O=C=O “bent” because we drew it on a V on paper?
4. Electron total for NH3.

Instruction

VSEPR theory (Chem 20). Electron domains around the central atom occupy space. A domain is a single, double, or triple bond (each counts as one domain) or a lone pair.

Five required shapes:

AXE	Domains	Shape name	Bond angle (ideal)	Examples
AX2	2	linear	180°	CO2, HCN, BeCl2
AX3	3	trigonal planar	120°	BF3, H2CO (approx.)
AX4	4	tetrahedral	109.5°	CH4, CCl4, NH4+
AX3E	4	trigonal pyramidal	~107°	NH3, PCl3
AX2E2	4	angular / V-shaped / bent	~104.5°	H2O
AX2E	3	angular / bent	~120°	SO2

Name angular, V-shaped, and bent as the same Chem 20 family (20-A2.5k wording).

Lone pairs occupy more space than bonding pairs, so NH3 and H2O angles are slightly less than 109.5°. CORE should state the ideal geometry name; PROFICIENT should mention the compression.

Multiple bonds: CO2 is AX2 (two domains), linear, even though each domain is a double bond. Do not call CO2 tetrahedral.

Ionic lattices have no VSEPR molecule shape. NaCl is not “linear.”

Timed visuals

id	trigger	duration	alt	teacher cue	tutor cue
CHEM20-VIS-07-DOMAIN	After retrieval	50 s	Balloon pairs (photo)	“Balloons = domains, including lone pairs.”	Do not require real balloons if latex allergy.
CHEM20-VIS-07-FIVE	Minute 14	70 s	Five-shape poster	Point to bent vs linear	Refuse CO2 as bent.
CHEM20-VIS-07-NH3	Minute 26	45 s	NH3 model, lone pair as a stick with no atom	“The lone pair is why it is not trigonal planar.”	—

Modelling/worked examples

Example 1 — CO2. Lewis O=C=O. Central C: 2 bonding domains, 0 lone pairs → AX2 → linear, 180°.

Example 2 — BF3. 24 e; three B–F singles (Chem 20 accepts BF3 as trigonal planar AX3; B has 6 e — exception named). Shape: trigonal planar, 120°.

Example 3 — CH4. AX4, tetrahedral, 109.5°. A flat square drawing is wrong.

Example 4 — NH3. Lewis: 3 bonding pairs + 1 lone pair on N → AX3E → trigonal pyramidal, not tetrahedral (the electron-pair geometry is tetrahedral; the molecular shape names the atom positions).

Example 5 — H2O. AX2E2 → bent. Electron-pair geometry tetrahedral; molecular shape bent.

Example 6 — SO2. Lewis (one acceptable Chem 20 form): S with a double bond, a single bond, and a lone pair → AX2E → bent.

Example 7 — CCl4 vs CHCl3. Both have four domains on C (no lone pairs) → tetrahedral electron/molecular shape. Polarity differs (Session 09); shape name is still tetrahedral.

Guided practice + answers

1. Shape of HCN. Answer: linear (AX2).
2. Shape of PCl3. Answer: trigonal pyramidal (AX3E).
3. Why is H2O not linear like CO2? Answer: O has two lone pairs (AX2E2); C in CO2 has none (AX2).
4. Shape of CCl4. Answer: tetrahedral.
5. Name the shape for AX3. Answer: trigonal planar.

Independent practice

• FOUNDATION: Complete a table: molecule, Lewis (given), AXE, shape for CO2, CH4, H2O, NH3.
• CORE: Predict without a given Lewis: BF3, H2S (like water, bent), PCl3, HCN, CCl4. Draw a 3-D wedge/dash for CH4.
• PROFICIENT: Distinguish electron-pair geometry vs molecular shape for NH3 and H2O. Explain why a double bond in CO2 counts as one domain. Predict SO2 as bent.
• ADVANCED: Build kit models (or paper tetrahedra) for CH4, NH3, H2O and photograph or sketch. One paragraph: why VSEPR does not apply to NaCl(s). Optional: H2CO (methanal) AX3 trigonal planar around C.

Project connection

Water is bent — that geometry is required for a polar molecule and for dissolving ionic cleaners (Session 09). CO2 is linear — polar bonds cancel. Record shapes of H2O, NH3 (if used as a banned household mixer with bleach — ammonia is not a project chemical). Ammonia is a VSEPR example only, not a selected cleaner.

Checks

Show CO2 card: students hold arms 180° vs 104°. If anyone “bends” CO2 because of the two oxygens, freeze and reteach domains.

Misconceptions

Wrong	Repair	Do not say
“Lewis drawing is the shape.”	Lewis is connectivity; VSEPR is 3-D.	“Yours looks artistic.”
“CO2 is bent like water.”	No lone pairs on C; AX2 linear.	—
“NH3 is tetrahedral because 4 pairs.”	Electron-pair geometry tetrahedral; molecular shape trigonal pyramidal.	“Same thing.”
“Triple bonds count as three domains.”	One domain.	—
“NaCl is linear VSEPR.”	Ionic lattice, no molecule.	—

AI Tutor prompts and boundaries

Allowed: “AXE and shape for H2O, NH3, CO2, CH4, BF3.” “Why is a double bond one domain?”

Forbidden: Inventing a boiling-water shape lab; ammonia+bleach; praising bent CO2; generating extra shapes (octahedral, T-shape) as CORE required.

Human-tutor handoff

Fine-motor: teacher-built models, student labels. ELL: keep the three bent synonyms posted. If balloon demo is used, non-latex only.

Exit ticket + answers

1. Shape of CH4.
2. Shape of H2O.
3. Shape of CO2.
4. AXE for NH3.

Answers: 1. tetrahedral. 2. bent / angular / V-shaped. 3. linear. 4. AX3E.

Homework

Draw wedge/dash sketches for the CORE list. Read: polar vs nonpolar teaser (one sentence in notebook: “shape + EN decide polarity”).

Teacher guidance

Grade the name of the molecular shape, not artistic talent. Require AXE on CORE. Do not distribute ammonia as a “real bottle” for the project.

Materials

Model kits or marshmallow/toothpick food items not to be eaten in lab; five-shape poster; wedge/dash handout. Marshmallows: if used, classroom models only, discarded, no eating (lab glassware contamination).

Preparation

Allergy check (latex balloons, marshmallow gelatin). Print AXE table. Count kit pieces.

Timing

Min	Move
0–6	Retrieval
6–20	VSEPR + five shapes + AXE
20–38	Worked models
38–50	Guided
50–66	Independent + kits
66–75	Exit

Online alternative

PhET *Molecule Shapes* (approved list) with a screenshot of CO2, H2O, NH3, CH4, BF3. Same table.

Low-technology alternative

Paper tetrahedron net; gumdrops forbidden if they will be eaten. Pipe cleaners: four arms, fold lone pairs down.