Using Molecular Model Kits in Class: A Practical Teaching Guide

Hand a class a box of model kit pieces and something changes. Students who glazed over at a page of dot-and-cross diagrams start arguing about which hole the next hydrogen should go in. Molecules stop being flat drawings and become objects that have a front, a back and an angle. That is the real value of model kits: they make the third dimension unavoidable.

They also bring their own problems. A kit can quietly teach that bonds are rigid sticks, that atoms come in bright colours, and that sodium chloride is made of little pairs. This guide sets out a single 60-minute lesson (with an optional follow-up) for students aged roughly 13–16, and treats the limitations of the models as part of the learning, not an afterthought.

What students should get out of it

By the end of the lesson students should be able to:

1. Build simple molecules from their dot-and-cross structures and explain why each atom has the number of bonds it does.
2. Describe the shapes of CH₄, H₂O, CO₂ and C₂H₄ and give approximate bond angles.
3. Compare a ball-and-stick model with a space-filling model and say what each shows well.
4. Build part of a sodium chloride lattice and explain why there is no “NaCl molecule”.
5. Name at least three ways in which physical models misrepresent real particles.

Prior knowledge: electron arrangements for the first 20 elements, and the idea of a covalent bond as a shared pair of electrons. If your class needs a refresher, the covalent bonding explainer is a quick read.

Before the lesson

• Kits. One kit between two or three students works best. Count out the pieces you need per group rather than handing over the whole box: roughly 3 carbon, 6 hydrogen, 2 oxygen and a handful of short and flexible links per group. For the lattice, you need either a dedicated ionic lattice kit or two colours of balls (at least 14 of each for a 3×3×3 cube) plus straight connectors.
• Colour key on the board. Most kits follow a common convention: carbon black, hydrogen white, oxygen red, nitrogen blue, chlorine green. Write it up so no one has to guess, and add a note that real atoms have no colour.
• A space-filling set, or images. If you do not have space-filling pieces, prepare a few images or a molecular viewer on the projector for the comparison section.
• Safety. Kit pieces are small and the links can be sharp at the ends. Keep them off the floor, don’t allow pieces near mouths, and count them back in at the end. No chemicals are used in this lesson.

Lesson outline (60 minutes)

0–8 min: starter, “draw it, then doubt it”

Ask students to draw a molecule of methane on mini whiteboards. Nearly all will draw a flat cross with carbon in the middle and four hydrogens at 90° to each other. Don’t correct it yet. Ask: “If the four bonding pairs repel each other, is 90° the furthest apart they could get?” Leave the question hanging.

8–20 min: build methane and water

Groups build CH₄. The carbon piece has four holes pointing towards the corners of a tetrahedron, so the kit forces the correct shape. Students then hold the model up and try to lay it flat on the desk. They can’t: at most three hydrogens touch the surface. Ask them to estimate the angle between two bonds. Reveal it is 109.5°, noticeably wider than the 90° in their starter drawing.

Now build H₂O. This is the first real teaching moment. If the kit’s oxygen piece has only two holes, point out that the model has hidden something. Oxygen has two lone pairs as well as two bonding pairs; four regions of electrons, just like carbon in methane. If your kit has four-hole oxygen pieces, add two short stubs (or small bits of modelling clay) to stand in for the lone pairs. The water molecule is then clearly methane with two hydrogens replaced by lone pairs, and the H–O–H angle closes to about 104.5° because lone pairs repel more strongly than bonding pairs.

20–30 min: double bonds with CO₂ and C₂H₄

Hand out the flexible links. For carbon dioxide, students join each oxygen to the carbon with two bent links each. The molecule should end up straight, with an O=C=O angle of 180°. Ask why: there are only two regions of electrons around the carbon, so they sit on opposite sides.

For ethene, C₂H₄, students join two carbons with a double bond (two flexible links) and add two hydrogens to each carbon. Ask them to try twisting one end relative to the other. With a single bond (try ethane, C₂H₆, if time allows) the ends rotate freely; with the double bond they cannot. That rigidity is real, and it is why ethene is flat, with bond angles of about 120° around each carbon. Link this to single, double and triple bonds for students who want more.

30–40 min: ball-and-stick versus space-filling

Put a ball-and-stick methane next to a space-filling version (a physical model or an image). Ask groups to list two things each model shows better.

	Ball-and-stick	Space-filling
Shows bond angles clearly	Yes	Hard to see
Shows which atoms are bonded	Yes, sticks make it obvious	Only by position
Shows relative atom sizes	Poorly; balls are often similar sizes	Better
Shows how much space the molecule takes up	No; mostly empty gaps	Yes
Shows how molecules might fit together	Poorly	Better
Easy to build and handle	Yes	Often fiddly

The point to land: neither model is “right”. Each is a tool that highlights some features and hides others. Chemists switch between them depending on the question.

40–52 min: the sodium chloride lattice

Groups (or the whole class, working together on one large model) build a cube of alternating sodium and chloride balls. In a 3×3×3 block, the centre ball is surrounded by six neighbours of the other colour: above, below, left, right, front and back.

Ask: “Which chloride belongs to the sodium in the middle?” The honest answer is none in particular; it is attracted to all six equally. That is why we write NaCl as a ratio, not as a molecule, and why the connectors in this model are misleading. They look like bonds between particular pairs, but the real attraction acts in all directions at once. The post on ionic lattice structure covers the arrangement in more detail.

If your kit uses equal-sized balls, point out that a chloride ion is actually considerably bigger than a sodium ion. A space-filling view shows the smaller sodium ions tucked into the gaps between the chloride ions.

52–60 min: plenary, “what did the model get wrong?”

Each group writes three ways their models misrepresent real particles. Collect answers on the board and add any missing items from the list in the next section. Return to the starter drawings: students redraw methane using wedge and dash lines to show the 3D shape.

Optional follow-up lesson

Extend to ammonia (NH₃, trigonal pyramidal, about 107°), ethyne (C₂H₂, linear) and a few molecules with more than four bonding regions for older students. The VSEPR practice questions make a good homework sheet afterwards.

What models get wrong

Build this list with the class rather than lecturing it:

• Atoms have no colour. The colour code is a convention, nothing more.
• Bonds are not sticks. A covalent bond is a shared pair of electrons, a region of negative charge between two nuclei, not a solid rod.
• Molecules are not rigid. Real bonds stretch and bend constantly, and molecules move and rotate all the time.
• Sizes are wrong. Ball-and-stick models shrink the atoms and exaggerate the gaps. Atoms in a molecule actually overlap.
• Lone pairs are usually invisible. Most kits give no way of showing them, so water looks like a “bent stick” for no apparent reason.
• Double bonds are not two identical bent springs. The two bonds in a double bond are different kinds, which older students meet later as sigma and pi bonds.
• Ionic lattices do not have connectors. The sticks in an NaCl model suggest pairwise bonds; the real attraction is non-directional.
• Real crystals are enormous. A grain of salt contains a vast number of ions, not 27.

Misconceptions to expect

• “Methane is flat.” Students default to what they can draw on paper. The kit fixes this quickly, so use it early.
• “Water is bent because oxygen is heavy” or “because the hydrogens are attracted to each other.” Steer them to repulsion between the four electron regions.
• “The double bond in CO₂ makes it bent.” Some students expect every molecule with oxygen to be bent like water. Compare the number of electron regions on the central atom.
• “There are NaCl molecules in salt.” The lattice model is the best cure, as long as you discuss the connectors.
• “Ball-and-stick is how molecules really look.” Pairing it with a space-filling model deals with this directly.

Questions to ask during the lesson

Pitch these at different depths to stretch the group:

1. Why does carbon always have four links in your model, and hydrogen only one?
2. How many regions of electrons are there around the oxygen in water? How many of them can you see in your model?
3. Why is CO₂ straight while H₂O is bent, even though both have a central atom bonded to two others?
4. What happens when you try to twist a double bond in your ethene model? Is that a real property?
5. In your NaCl model, how many chloride ions surround each sodium ion? Why is it misleading to connect them with sticks?
6. Which model would you use to show that methane is roughly spherical? Which would you use to measure a bond angle?
7. Name one thing your model shows correctly and one thing it gets wrong.

Adapting the lesson

• No kits? Use modelling clay and cocktail sticks for covalent molecules (supervise the sharp ends, or use pipe cleaners instead), and polystyrene balls glued in layers for the lattice.
• Lower-attaining groups: stick to CH₄, H₂O and the lattice, and give a printed colour key and a build card for each molecule.
• Higher-attaining groups: ask them to predict the shape before building, then check. Add NH₃ and ask them to explain the angle between methane’s and water’s.
• Homework: redraw each molecule with wedges and dashes, and write a short paragraph on the limitations of models.

For a wider sequence that places this lesson within a whole bonding topic, see Teaching Chemical Bonding.

Key takeaways

• Model kits are most valuable for making molecular shape three-dimensional and memorable.
• Build CH₄ first (tetrahedral, 109.5°), then H₂O (bent, about 104.5°) to show the effect of lone pairs, then CO₂ (linear) and C₂H₄ (planar, rigid double bond).
• Compare ball-and-stick and space-filling models side by side: each shows different features.
• The NaCl lattice model is the clearest way to show that ionic compounds are giant structures, not molecules.
• Always finish by asking what the models get wrong. Understanding limitations is part of the specification, and it prevents misconceptions from being built in plastic.