🧰 You need: pen, pencil, ruler, calculator, graph paper or squared paper. Time: 45 minutes. Course: Combined and Separate.
Information – read this first
| Date | Model | What to draw | Key idea |
|---|---|---|---|
| Early 1800s | Dalton: solid sphere | A plain shaded circle | Atoms are tiny spheres that cannot be divided. |
| 1897 | Thomson: plum pudding | A large circle marked + with small − electrons dotted inside | The electron was discovered. The atom is a ball of positive charge with negative electrons embedded in it. |
| 1909–1911 | Rutherford: nuclear model | A tiny dot (nucleus, +) in the centre, electrons around the outside, lots of empty space | Alpha scattering experiment: most alpha particles went straight through, so the atom is mostly empty space. A few bounced back, so the positive charge and mass are concentrated in a tiny nucleus. |
| 1913 | Bohr: electron shells | A nucleus with electrons on circular rings (shells) at fixed distances | Electrons orbit the nucleus at specific distances. |
| 1932 | Chadwick: the neutron | A nucleus drawn as + protons and neutral neutrons, with shells around it | Chadwick showed that the nucleus also contains neutrons. |
- Proton: charge +1, relative mass 1, in the nucleus. Neutron: charge 0, relative mass 1, in the nucleus. Electron: charge −1, very small mass, in shells around the nucleus.
- An atom has no overall charge because it has equal numbers of protons and electrons.
- Atomic number = number of protons. Mass number = protons + neutrons.
- A radioactive nucleus is unstable and gives out radiation at random.
- Half-life is the time it takes for the number of unstable nuclei in a sample to halve, or for the count rate to fall to half.
The data: a radioactive sample
| Time (min) | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| Count rate (counts per minute) | 800 | 566 | 400 | 283 | 200 | 141 | 100 |
What to do
- (2 min) Write the title Models of the Atom and today’s date. Underline both with a ruler.
- (15 min) Draw a timeline: rule a long arrow across your page (or down it). Mark the five dates from the table. At each date draw the model (about 3 cm across), write the scientist’s name and copy the key idea in your own words.
- (5 min) Draw a large atom using the modern model. Label: nucleus, proton (+), neutron (0), electron (−), shell. Then copy the proton/neutron/electron facts.
- (10 min) Plot a half-life graph:
• Time (min) along the bottom from 0 to 6. Count rate up the side from 0 to 800.
• Plot the points with small crosses and draw a smooth curve through them (not straight lines).
• Draw a dashed line across from 400 to the curve, then down to the time axis. Read off the half-life. - (10 min) Write the heading Calculations. Copy the worked example, then do Q1–Q5.
- (3 min) Check your answers and correct in a different colour.
Calculations
Worked example: A sample has a count rate of 800 counts per minute. What is the count rate after 3 half-lives?
800 → 400 (1) → 200 (2) → 100 counts per minute (3)
Q1 What is the half-life of the sample in your graph?
Show answer
2 minutes (800 falls to 400 in 2 minutes, and 400 falls to 200 in the next 2 minutes).
Q2 A sample has a count rate of 1600 counts per minute. What is it after 2 half-lives?
Show answer
1600 → 800 → 400 counts per minute
Q3 An isotope has a half-life of 5 years. What fraction of it is left after 20 years?
Show answer
20 ÷ 5 = 4 half-lives. ½ × ½ × ½ × ½ = 1/16
Q4 Carbon-14 has atomic number 6 and mass number 14. How many protons, neutrons and electrons does a carbon-14 atom have?
Show answer
6 protons, 8 neutrons (14 − 6) and 6 electrons.
Q5 A sample starts with 6000 unstable nuclei. Its half-life is 3 hours. How many unstable nuclei are left after 9 hours?
Show answer
9 ÷ 3 = 3 half-lives. 6000 → 3000 → 1500 → 750
⭐ Challenge
Explain how the results of the alpha scattering experiment showed that the plum pudding model was wrong.
Show answer
In the plum pudding model the positive charge is spread out, so all the alpha particles should have passed straight through with very little deflection. In fact, some were deflected through large angles and a few bounced straight back. This could only happen if the positive charge and most of the mass were concentrated in a tiny nucleus. Most passed straight through, showing the atom is mostly empty space.
