Atoms, nuclear radiation and half-life
Some nuclei are unstable and give out radiation to become more stable. You need to know what is in an atom, the three main types of radiation and how to use half-life, which is a favourite calculation.
Part 1 of 3: Learn it
In short
- Alpha is a helium nucleus, beta is a fast electron from the nucleus, gamma is an electromagnetic wave.
- Alpha is the most ionising and the least penetrating; gamma is the opposite.
- Half-life is the time for half the unstable nuclei in a sample to decay, or for the count rate to halve.
Where this is in your specification
Spec points: AQA 4.4.1 and 4.4.2, Edexcel Topic 6, OCR P6.1
| Board | Topic: Atomic structure and radioactivity |
|---|---|
| AQA 8463 | 4.4 (8463); 6.4 (8464) |
| Edexcel 1PH0 | Topic 6 |
| OCR J249 | P6 and P4 |
Inside the atom
An atom has a tiny, positive nucleus of protons and neutrons, with electrons in energy levels around it. The radius of an atom is about 1 × 10⁻¹⁰ m, and the nucleus is roughly 10 000 times smaller, yet it holds almost all the mass.
The model changed as evidence arrived: the plum pudding model was replaced after the alpha scattering experiment showed most alpha particles pass straight through gold foil while a few bounce back, so the mass and charge must be concentrated in a small nucleus.
- Atomic number: the number of protons. It decides the element.
- Mass number: protons plus neutrons.
- Isotopes: atoms of the same element (same number of protons) with different numbers of neutrons.
Alpha, beta and gamma
| Alpha (α) | Beta (β) | Gamma (γ) | |
|---|---|---|---|
| What it is | 2 protons + 2 neutrons (a helium nucleus) | a high-speed electron from the nucleus | electromagnetic radiation |
| Stopped by | a sheet of paper or a few cm of air | a few mm of aluminium | reduced by thick lead or concrete |
| Ionising | strongly | moderately | weakly |
| Effect on the nucleus | mass number − 4, atomic number − 2 | atomic number + 1 (a neutron becomes a proton) | no change in mass or atomic number |
Half-life
Radioactive decay is random: you cannot say when one nucleus will decay. But a large sample decays at a predictable rate. The half-life is the time it takes for the number of unstable nuclei, or the count rate, to fall to half.
On a graph of count rate against time, read off the time for the count to halve, then check it again from a different starting point: it should be the same.
Contamination and irradiation
Irradiation is being exposed to radiation from a source outside you; it stops when you move away and does not make you radioactive. Contamination is when radioactive atoms get onto or into something, so it keeps being exposed until they are removed. Inside the body alpha is the most dangerous; outside it, gamma and beta are.
What fraction of a sample is left after 4 half-lives?
Show the answer
1/16 (½ × ½ × ½ × ½).
Part 2 of 3: See it worked
Worked examples
Example 1
A sample has a count rate of 800 counts per minute and a half-life of 3 hours. What is the count rate after 9 hours?
- 9 hours ÷ 3 hours = 3 half-lives
- 800 → 400 → 200 → 100
- Count rate after 9 hours = 100 counts per minute
Answer: 100 counts per minute (ignoring background radiation).
Example 2
Uranium-238 (atomic number 92) decays by alpha emission. What are the mass number and atomic number of the new nucleus?
- Alpha takes away 2 protons and 2 neutrons
- Mass number: 238 − 4 = 234
- Atomic number: 92 − 2 = 90
Answer: Mass number 234, atomic number 90 (thorium).
Common mistakes
- Saying beta particles come from the electron shells. They come from the nucleus.
- Thinking a whole sample has gone after two half-lives. After two half-lives a quarter is left.
- Mixing up contamination and irradiation.
- Forgetting to subtract background radiation from a measured count when a question gives it.
Which radiation is stopped by paper?
Show the answer
Alpha.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What fraction of a sample is left after 4 half-lives?
1/16 (½ × ½ × ½ × ½).
Which radiation is stopped by paper?
Alpha.
Carbon-12 and carbon-14 are isotopes. What is the same and what is different?
Both have 6 protons; carbon-14 has 2 more neutrons.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses