The nucleus of an atom
An atom has a tiny, dense nucleus of protons and neutrons (together called nucleons), surrounded by electrons. The atomic (proton) number Z is the number of protons. The mass (nucleon) number A is the number of protons plus neutrons. Isotopes are atoms of the same element (same Z) with different numbers of neutrons, such as carbon-12 and carbon-14.
Natural radioactivity
Some nuclei are unstable and give out radiation to become more stable. This is radioactive decay. It is random (we cannot say when a particular nucleus will decay) and spontaneous (it is not affected by temperature, pressure or chemical reactions). There are three kinds of radiation.
| Alpha (α) | Beta (β) | Gamma (γ) | |
|---|---|---|---|
| Nature | Helium nucleus (2 protons + 2 neutrons) | A fast electron from the nucleus | Electromagnetic wave of very short wavelength |
| Charge | +2 | −1 | 0 |
| Penetration | Stopped by paper or a few centimetres of air | Stopped by a few millimetres of aluminium | Reduced by thick lead or concrete |
| Ionising power | Strongest | Medium | Weakest |
| Deflected by magnetic field? | Yes (slightly) | Yes (strongly, opposite way) | No |
| Effect on the nucleus | A decreases by 4, Z by 2 | A unchanged, Z increases by 1 | No change in A or Z |
| Decay | Equation example |
|---|---|
| Alpha | ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He |
| Beta | ¹⁴₆C → ¹⁴₇N + ⁰₋₁e |
| Gamma | Follows α or β decay when the nucleus is left excited |
Half-life
- Half-life
- The time taken for half of the radioactive nuclei in a sample to decay. It is constant for a given isotope.
Choose the half-life and the time that has passed. Find the fraction and percentage of the sample left.
A sample of a radioactive isotope has a half-life of 6 hours. What fraction is left after 24 hours?
- 24 hours is 4 half-lives.
- Each half-life halves the amount.
Answer: One-sixteenth (6.25%) is left.
Artificial radioactivity and uses
Stable nuclei can be made radioactive by bombarding them with neutrons in a nuclear reactor or with particles from an accelerator. These radioisotopes have many uses.
| Use | Radioisotope and idea |
|---|---|
| Medical tracers | Iodine-131 to study the thyroid; technetium-99m for scans |
| Treating cancer | Cobalt-60 gamma rays destroy cancer cells (radiotherapy) |
| Sterilising instruments and food | Gamma rays kill germs |
| Thickness gauges | Beta particles passing through paper or plastic; fewer pass through if the sheet is thicker |
| Smoke alarms | Americium-241 alpha source; smoke reduces the ionisation current |
| Dating | Carbon-14 half-life 5 730 years dates once-living objects; uranium dates rocks |
| Leak tracing | A tracer in a pipe is detected where it leaks out |
Radiation hazards and safety
Radiation ionises atoms and can damage cells, causing burns, cancer and genetic damage. Everybody receives a little background radiation from rocks, radon gas, cosmic rays and food. Safety rules: use the smallest dose, keep the longest distance, and spend the shortest time; shield with lead; handle sources with tongs, never by hand; store in lead containers; wear a film badge to measure the dose.
Nuclear fission and fusion
| Fission | Fusion | |
|---|---|---|
| What happens | A heavy nucleus (uranium-235) splits into two smaller nuclei and more neutrons | Two light nuclei (hydrogen isotopes) join to make helium |
| Energy | Large energy per atom | Even more energy per kilogram |
| Conditions | Slow neutrons; chain reaction controlled in a reactor by boron or cadmium rods | Millions of degrees (in the Sun) |
| Use | Nuclear power stations, atomic bombs | The Sun and stars; research reactors |
| Waste | Long-lived radioactive waste | Little long-lived waste |
Key terms
- Isotopes
- Atoms with the same number of protons but different numbers of neutrons.
- Half-life
- The time for half of the radioactive nuclei in a sample to decay.
- Fission
- The splitting of a heavy nucleus into two lighter nuclei.
- Fusion
- The joining of two light nuclei into one heavier nucleus.
Summary
- Radioactive decay is random and spontaneous; α, β and γ differ in nature, penetration and ionisation.
- Half-life: N = N₀ (½)^(t/T).
- Radioisotopes are used in medicine, industry and dating; radiation is hazardous and needs shielding.
- Fission splits heavy nuclei; fusion joins light nuclei; E = mc².