← Back to Atomic Structure contents
ATOMIC STRUCTURE · CHAPTER 4 OF 12
What is radioactive decay?
Some nuclei stay unchanged. Others can change without warning, giving out radiation. What changes, what leaves the nucleus, and why can’t we predict the next decay?
By the end, you’ll be able to…
- explain radioactive decay using unstable nuclei, randomness and spontaneous change
- identify alpha, beta minus, beta plus, gamma and neutron radiation
- explain ionisation and compare alpha, beta and gamma radiation
- describe activity, use becquerels and distinguish activity from count rate.
It starts in the nucleus
Some isotopes have unstable nuclei. An unstable nucleus can change and emit radiation, releasing energy and moving towards a more stable state. This process is called radioactive decay.
The change may produce a different nucleus or leave the nucleus in a lower-energy state. A product can still be unstable and decay again.
Radioactive decay is spontaneous: it does not need an external trigger. Ordinary changes in temperature, pressure or chemical conditions do not change the decay probability of the isotope. It is also random: we cannot predict exactly when a particular nucleus will decay.
Same element. Different stability.
Compare the two carbon isotopes, then reveal which one is radioactive.
Carbon-12
6 protons · 6 neutrons
Stable or radioactive?
Carbon-12 is stable. Its nucleus does not undergo radioactive decay.
Carbon-14
6 protons · 8 neutrons
Stable or radioactive?
Carbon-14 is radioactive. Its unstable nucleus can undergo beta-minus decay. The nuclear change is explored in Chapter 5.
* Pink + particles are protons; white particles are neutrons. The faint circles indicate approximate nuclear extent, not a wall. Diagrams are not to scale. There is no simple rule that every isotope with more neutrons is radioactive.
Which nucleus will decay next?
Choose a nucleus to watch. Select “Observe the sample”. A yellow halo and “D” mark nuclei that have decayed; “U” means undecayed.
What should you notice?
Nuclei do not decay in a queue or at evenly spaced times. Identical undecayed nuclei do not come with individual countdown clocks.
For a particular radioactive isotope, an undecayed nucleus has a constant probability of decay per unit time. Large samples show predictable overall patterns, even though individual decays are unpredictable. We explore half-life in Chapter 7.
* This is a scripted illustration of irregular timing, not a random-number simulation. Reset repeats the same sequence. The symbols stand for nuclei; emitted radiation and changes inside each nucleus are omitted. Decayed nuclei remain present.
What can leave an unstable nucleus?
Nuclear radiation may be particles or electromagnetic radiation. Alpha, beta and gamma radiation all come from changes in the nucleus. They are not the same thing as an outer electron changing shell.
Meet the emissions
Select a type of radiation. Watch what leaves the nucleus and how the nucleus changes after emission.
Alpha decay: a helium nucleus leaves
Two protons and two neutrons leave together. Relative charge +2; relative mass 4.
Beta-minus decay: a neutron becomes a proton
A high-speed electron is emitted. Relative charge −1; relative mass approximately 1/1836.
Beta-plus decay: a proton becomes a neutron
A high-speed positron is emitted. Relative charge +1; relative mass approximately 1/1836.
Gamma emission: the nucleus loses energy
Gamma radiation has no electric charge and no rest mass.
Neutron emission: one neutron leaves
An uncharged neutron is emitted. Relative charge 0; relative mass approximately 1.
* The nucleus remains in the same position while its composition changes. Each animation cycle represents a new decay event, not repeated decay of one nucleus. Small symbolic nuclei illustrate the counting rules; they are not verified decay examples for particular isotopes. Speeds, sizes and distances are not to scale.
Radiation can ionise atoms
Alpha, beta minus, beta plus and gamma radiation are ionising. They can transfer enough energy to remove an electron from an atom. Losing a negative electron leaves a positive ion.
Ionisation changes an atom’s electrons. Radioactive decay changes its nucleus. These are different processes.
Remove an electron. What is left?
Select the button to show an electron removed by energy transferred from ionising radiation. Select it again to reset.
* A simplified hydrogen example. The energy-transfer event is omitted; the moving electron illustrates the outcome. Not to scale.
Which radiation gets through?
Alpha particles produce many ionisations over a short distance, so they lose energy quickly. Beta particles are less strongly ionising and penetrate farther. Gamma rays are weakly ionising compared with alpha and beta radiation, but are much more penetrating.
Add a barrier
Select a barrier. Compare how far each line continues. A faded gamma line means reduced intensity, not slower radiation.
* The diagram compares typical behaviour; penetration depends on radiation energy and absorber thickness. It does not show particles to scale. Thick concrete can also reduce gamma intensity.
Compare their properties
| Radiation | Ionising power | Typical range in air | Shielding |
|---|---|---|---|
| Alpha | Strong | A few centimetres | Paper or the outer dead layer of skin |
| Beta | Medium | Up to a few metres | A few millimetres of aluminium |
| Gamma | Weak compared with alpha and beta | Long range; intensity decreases with distance and absorption | Thick lead or concrete reduces intensity |
* These are approximate comparisons, not fixed distances for every source. Beta includes beta-minus electrons and beta-plus positrons; their detailed interactions can differ.
How active is a source?
Activity is the number of nuclear decays per second in a source. Its unit is the becquerel (Bq). An activity of 1 Bq means one decay per second on average.
Activity refers to the source. Count rate is the number of counts registered by a detector per second (or per minute). A detector usually records only some of the radiation emitted, and it may also detect background radiation. Its count rate is not automatically equal to the source’s activity.
To calculate average activity from a known number of decays:
Because decay is random, measured counts fluctuate. Average over an interval rather than expecting exactly the same number every second.
Three worked examples
These examples give the actual number of nuclear decays, rather than a detector’s recorded counts.
Calculate average activity:
240 decays occur in 60 s. Calculate the average activity.
Average activity = number of decays ÷ time
Average activity = 240 ÷ 60
Average activity = 4 Bq
Calculate average activity:
900 decays occur in 30 s. Calculate the average activity.
Average activity = number of decays ÷ time
Average activity = 900 ÷ 30
Average activity = 30 Bq
Calculate average activity:
1500 decays occur in 1 minute. Calculate the average activity. Use 60 s.
Average activity = number of decays ÷ time
Average activity = 1500 ÷ 60
Average activity = 25 Bq
Does 200 Bq mean 200 detector counts?
Check your understanding
Choose one answer for each question. Feedback appears underneath.
“A source is 50 Bq, so the detector must click exactly 50 times every second. Heating it will make its nuclei decay faster.” Explain the mistakes.
Use activity, detection, randomness and spontaneous decay in your explanation.
Reveal the answer
50 Bq means 50 nuclear decays per second on average. Randomness means the number in a particular second can vary.
A detector does not necessarily register every emission. Its reading depends on the arrangement and detection efficiency, and background radiation can add counts.
Radioactive decay does not need heating to trigger it. For GCSE purposes, ordinary changes in temperature, pressure or chemical conditions do not change the decay probability of the isotope.
CHAPTER 4
Chapter summary
- Unstable nuclei undergo spontaneous, random radioactive decay and emit radiation.
- Individual decay times cannot be predicted, but large samples show predictable patterns.
- Alpha particles are helium nuclei; beta minus particles are electrons; beta plus particles are positrons.
- Gamma radiation is electromagnetic radiation. Some nuclei emit neutrons.
- Ionising radiation can remove electrons from atoms. This differs from changing the nucleus.
- Alpha is strongly ionising and weakly penetrating; gamma is less strongly ionising and more penetrating.
- Activity is decays per second, measured in Bq. Detector count rate is a different measurement.