Unit 9: The Development of the Atomic Model
Scientific models are not invented once and never changed — they develop as new evidence appears. This unit traces how our model of the atom evolved, and introduces radioactivity.
01 How the atomic model changed
| Scientist | Model |
|---|---|
| Dalton (early 1800s) | Atoms are tiny, solid, indivisible balls |
| Thomson (1897) | "Plum pudding" model — a positive sphere with negative electrons scattered inside |
| Rutherford (1911) | Nuclear model — a tiny, dense, positive nucleus with electrons orbiting far away, mostly empty space (from the gold foil experiment) |
| Bohr (1913) | Electrons orbit the nucleus in fixed energy levels (shells) |
Key idea: each new model was accepted because it explained new experimental evidence that the older model could not explain. This is how science develops.
02 Radioactivity
Some atoms have unstable nuclei. They release energy and particles to become more stable — this is called radioactive decay. There are three common types of radiation:
| Type | What it is | Stopped by |
|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons | A sheet of paper |
| Beta (β) | A fast-moving electron | A few mm of aluminium |
| Gamma (γ) | High-energy electromagnetic wave | Thick lead or concrete |
03 Half-life
The half-life of a radioactive substance is the time it takes for half of the radioactive nuclei in a sample to decay. It is used in dating ancient objects and in medical treatments.
V Key vocabulary
| Term | Meaning |
|---|---|
| Radioactive decay | The random process by which an unstable nucleus releases radiation to become more stable. |
| Half-life | The time taken for half of the radioactive nuclei in a sample to decay. |
| Nucleus | The dense, positively charged centre of an atom (Rutherford's discovery). |
| Isotope | Atoms of the same element with different numbers of neutrons. |
? Quick check
Try to answer before you tap. Answers are hidden underneath.
What evidence from the gold foil experiment led Rutherford to reject the plum pudding model?
Most alpha particles passed straight through the foil, but a few bounced back strongly. This showed atoms are mostly empty space with a tiny, dense, positively charged nucleus, not a spread-out positive sphere.
Which type of radiation is stopped by a single sheet of paper?
Alpha radiation.
A sample has a half-life of 10 years. How much of the original radioactive material remains after 30 years?
One eighth (1/8) remains — three half-lives have passed (½ × ½ × ½ = 1/8).