Atomic Structure (Physics)
Radioactivity, nuclear decay, half-life, and uses of radiation.
In this topic
Radioactive Decay
Some atomic nuclei are unstable and emit radiation to become more stable. This is radioactive decay — it is random and spontaneous.
Types of radiation: - Alpha (α): 2 protons + 2 neutrons (helium nucleus). Strongly ionising, stopped by paper, short range in air (~5cm). Mass number decreases by 4, atomic number by 2. - Beta (β): high-speed electron emitted when a neutron turns into a proton. Moderately ionising, stopped by aluminium, range ~1m in air. Atomic number increases by 1. - Gamma (γ): electromagnetic wave. Weakly ionising, reduced by thick lead/concrete, very long range. No change in mass or atomic number.
Half-life: the time taken for the number of radioactive nuclei (or activity) to halve.
Key Points
- Radioactive decay is random — you cannot predict when a specific nucleus will decay
- Half-life is constant for a given isotope
- After n half-lives, fraction remaining = (½)ⁿ
- Uses: smoke detectors (alpha), medical tracers (gamma), radiotherapy (gamma)
Example Questions
2A radioactive source has an activity of 800 Bq. Its half-life is 3 hours. What is the activity after 9 hours?
9 hours = 3 half-lives. 800 → 400 → 200 → 100 Bq
[2 marks]
3Explain why alpha particles are used in smoke detectors rather than beta or gamma radiation.
Alpha particles are strongly ionising, so they ionise the air effectively, creating a current. When smoke enters, it absorbs the alpha particles, the current drops, and the alarm sounds. Alpha is also stopped by a few centimetres of air, so it is safe — it won't pass through the detector casing and harm people.
[3 marks]