Quick answer: An atom has a tiny nucleus of protons and neutrons, with electrons in shells around it. This Chemistry 9-10 lab on atomic structure gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How can a handful of two kinds of bolt model the isotopes of an element? You will weigh mixed handfuls, compare the measured mean mass with a calculated weighted mean, and see why a relative atomic mass is an average that matches no single atom.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-1, GCSE C1
Aim
To model isotopes with two sizes of bolt and to show that the average mass of a mixed handful is a weighted mean, just like a relative atomic mass.
Equipment
- Top-pan balance reading to 0.1 g
- About 40 small bolts and 20 large bolts
- Paper cup or tray
- Calculator and graph paper
Safety
Keep bolts off the floor so nobody slips on them. Do not put them in your mouth. Wash your hands after handling metal items.
Method
- Weigh 10 small bolts and 10 large bolts separately. Divide each mass by 10 to get the mass of one bolt.
- Take a random handful of 40 bolts without looking. Count how many are small and how many are large.
- Place all 40 bolts on the tray and record the total mass. Divide by 40 to find the mean mass per bolt.
- Repeat steps 2 and 3 for four different handfuls.
Results
Mass of 10 small bolts = 40.2 g, so one small bolt = 4.02 g. Mass of 10 large bolts = 59.8 g, so one large bolt = 5.98 g.
| Handful | Small bolts | Large bolts | Total mass / g | Mean mass per bolt / g |
|---|---|---|---|---|
| 1 | 28 | 12 | 184.4 | 4.61 |
| 2 | 25 | 15 | 190.1 | 4.75 |
| 3 | 31 | 9 | 178.5 | 4.46 |
| 4 | 22 | 18 | 196.0 | 4.90 |
Analysis questions
Question 1 Predict handful 1
Use the masses of single bolts to calculate the expected mass of handful 1 and its mean mass per bolt.Question 2 Compare with measurements
Do the same for handfuls 2, 3 and 4. State the largest difference between a calculated and a measured total mass.Question 3 Plot the graph
Calculate the fraction of large bolts in each handful. Plot mean mass per bolt (y) against fraction of large bolts (x) and draw a line of best fit.Question 4 Meaning of intercept and gradient
Find the gradient and the y-intercept of your line. State what each one represents.Question 5 Link to isotopes
State which part of the model matches (a) an isotope, (b) percentage abundance, (c) relative atomic mass.Question 6 Not a bolt mass
Explain why the mean mass of 4.61 g is not the mass of any single bolt. Give a real chemical example of this.Question 7 Balance uncertainty
The balance reads to ±0.1 g. Calculate the percentage uncertainty in the total mass of handful 1 and in the mean mass per bolt.Question 8 Estimate an unknown mixture
A mystery handful of 40 bolts has a total mass of 192.0 g. Use your graph relationship to estimate the fraction of large bolts, and suggest one way to improve the experiment.
