Why we use molecular models
Atoms are far too small to see. A model is a simple copy that keeps the important facts and drops the rest. With a model you can answer questions like: which atoms are joined? What angle is between the bonds? Will two molecules fit together?
To use a model in a task, follow three steps: (1) write the formula and draw the Lewis structure, (2) build or open the model, (3) read the shape, the angles and the size from it.
Ball-and-stick and space-filling models
Ball-and-stick: balls are atoms, sticks are bonds. Double bonds use two sticks. It shows bond lengths and angles clearly, but the balls are drawn smaller than real atoms.
Space-filling: the balls are as large as the atoms really are, and they touch or overlap. You see the outside surface and how much room the molecule takes. You cannot see the bonds well.
Computer models can show both, rotate smoothly, colour the surface by charge and measure angles and distances. The colours are standard: C black or grey, H white, O red, N blue.
Predicting the shape from a model
Electron pairs around the central atom repel each other, so they spread out as far as possible. Count the pairs (bond pairs + lone pairs):
- 2 pairs: straight line, 180 degrees (CO₂).
- 3 pairs: flat triangle, 120 degrees.
- 4 pairs, no lone pair: tetrahedron, 109.5 degrees (CH₄).
- 4 pairs, 1 lone pair: pyramid, about 107 degrees (NH₃).
- 4 pairs, 2 lone pairs: bent, about 104.5 degrees (H₂O).
Lone pairs push harder than bond pairs, so the angle becomes a little smaller. The shape names only the atoms, not the lone pairs.
Building a reaction mechanism
A mechanism shows how a reaction happens, step by step: which bonds break, which form, and in which order. Curved arrows show where a pair of electrons moves.
Example, SN2: OH⁻ attacks the carbon in CH₃Br from the side opposite to Br. In one single step the C-O bond forms while the C-Br bond breaks. In the middle, the carbon is joined partly to both and the three H atoms lie in one plane. At the end the H atoms have flipped like an umbrella in the wind. This flip is called inversion.
When a reaction has several steps, the slowest step is the rate-determining step. A model of each step helps you see which one is slowest.
Catalysis inside a mechanism
A catalyst gives the reaction a new path with a lower energy barrier. In a mechanism it takes part in one step and is given back in a later step, so it is not used up. In a model you can see why: the catalyst surface or molecule holds the reactants in the right position, so they meet in the right orientation.
Try it
No kit? Use 5 grapes or sweets as atoms and toothpicks as bonds. Build CH₄ with 4 toothpicks on one central ball. Try to put all 4 in one flat plane. Does it look natural? Spread them out and see the tetrahedron appear. In the 3D above, pick a molecule, guess its shape, and then check.
Key formulas and definitions
- Electron pairs = bond pairs + lone pairs on the central atom
- 2 pairs: 180°; 3 pairs: 120°; 4 pairs: 109.5° (less with lone pairs)
- Sticks in an alkane CnH(2n+2): 3n + 1
- SN2: Nu⁻ + R–X → R–Nu + X⁻ (one step, inversion)
- Colours: C black, H white, O red, N blue
Worked examples
1. Predict the shape of CO₂ and say why.
Carbon has 2 double bonds and no lone pair, so there are 2 electron regions. They spread to 180 degrees. CO₂ is linear.
2. Why is NH₃ a pyramid and not flat?
Nitrogen has 3 bond pairs and 1 lone pair, so 4 pairs in all. They form a tetrahedron, and the 3 H atoms make its base. The lone pair pushes the H atoms down, angle about 107 degrees.
3. A ball-and-stick model of ethane (C₂H₆) is built. How many balls and sticks are needed?
Balls: 2 C + 6 H = 8. Sticks: 1 C–C + 6 C–H = 7.
4. In the SN2 reaction CH₃Br + OH⁻, why does OH⁻ attack from the side opposite to Br?
Br is a big atom with partial negative charge, and OH⁻ is also negative. Coming from the opposite side keeps them far apart and lets the C–Br bond break as C–O forms. This causes inversion of the three H atoms.
5. Which model would you use to judge whether a molecule fits into a small pocket, and why?
A space-filling model. Its balls show the true size, so you can see if the molecule fits.
Common mistakes
- Thinking the balls in a ball-and-stick model show real atom size. They are made small so that sticks can be seen.
- Counting a double bond as one stick. Draw two sticks, but treat it as ONE electron region when predicting shape.
- Forgetting the lone pairs. Water has 4 electron pairs, so it is bent (not straight) even though it has only 3 atoms.
- Believing a model is the real molecule. It is a simplified picture; real molecules vibrate and have no hard edges.