Quick answer: An atom is the smallest particle of an element, and a molecule is two or more atoms bonded together. This grade 8 science lab on atoms and molecules gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How can we check a chemical formula by building a model? In this lab you build six molecules from clay and sticks, count their atoms and bonds, and weigh each model to see whether the number of balls agrees with the formula.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-1
Aim
To build models of six simple molecules, count their atoms and bonds, and check each against its chemical formula.
Equipment
- Modelling clay in four colours (navy or black for carbon, white for hydrogen, red for oxygen, violet or blue for nitrogen)
- About 30 cocktail sticks
- A kitchen balance reading to 0.1 g
- Ruler and paper
Safety
Cocktail sticks have sharp ends. Push them in carefully, keep them away from your face and put them in the bin when finished. Wash your hands after handling clay and never put it in your mouth.
Method
- Roll balls of clay of about equal size, 3 g each. Use the balance to check a few.
- For each molecule in the table, choose the right number of balls of each colour.
- Join the balls with sticks: one stick for a single bond, two sticks side by side for a double bond (O₂ and CO₂).
- Count the balls and the sticks used and record them.
- Place each finished model on the balance and record its mass.
- Sketch each model before you take it apart.
Results
A group in Colombo recorded the sample data below. The balls were about 3 g each.
| Molecule | Formula | Balls used | Sticks used | Mass of model (g) |
|---|---|---|---|---|
| Hydrogen | H₂ | 2 | 1 | 6.1 |
| Oxygen | O₂ | 2 | 2 | 6.0 |
| Water | H₂O | 3 | 2 | 9.2 |
| Carbon dioxide | CO₂ | 3 | 4 | 8.9 |
| Methane | CH₄ | 5 | 4 | 15.1 |
| Ammonia | NH₃ | 4 | 3 | 12.0 |
Analysis questions
Question 1 Atoms from formulas
State how the number of balls used for each molecule can be found from its formula. Check this for CH₄ and NH₃.Question 2 Mass per ball
Calculate the mass per ball (model mass ÷ balls used) for each molecule, then the mean of the six values.Question 3 Spread of the results
State the highest and lowest mass per ball and calculate the range. Suggest one reason why the values are not identical.Question 4 Sticks and valency
Look at the number of sticks touching each atom type in your models. State how many sticks touch carbon, hydrogen, oxygen and nitrogen atoms.Question 5 Predict ethanol
Ethanol has the formula C₂H₆O. Predict the number of balls and of sticks you would need and the mass of the model if balls are 3.0 g.Question 6 Limits of the model
Describe two ways in which your clay models are different from real molecules.Question 7 Improving the lab
Suggest two improvements that would make the mass measurements more reliable.
