Radioactivity basics for ICSE Class 10 Physics
Ever wonder why old watches keep ticking long after they’re built? The secret lies in the quiet glow of radioactive decay.
💡 In Simple Words: Radioactivity is when an unstable atom’s centre (the nucleus) spits out tiny bits or energy to become stable. Those bits are called radiation.
What is radioactivity?
Every atom has a centre called a nucleus (think of it as the sun in a tiny solar system). Some nuclei are perfectly happy, but others are like a crowded room – they want to get rid of extra particles. When they do, they release radiation and become a different, more stable atom.
Why do some atoms become unstable?
Atoms that have the same number of protons (positively charged particles) but different numbers of neutrons (neutral particles) are called isotopes. If the neutron‑to‑proton ratio is too high or too low, the nucleus feels “unbalanced” and may break apart – that’s radioactivity.
Types of radiation
Radioactive atoms can release three main kinds of radiation. Here’s a quick comparison:
| Radiation | Mass | Charge | Penetrating power | Common shielding |
|---|---|---|---|---|
| Alpha (α) particle | Heavy (like a helium nucleus) | Positive | Very low – stops in a sheet of paper | Paper, skin |
| Beta (β) particle | Light (an electron or a positron) | Negative or positive | Medium – can pass through paper but stopped by thin metal | Aluminium foil |
| Gamma (γ) ray | None – pure energy | Neutral | High – needs thick lead or concrete | Lead, concrete |
Think of alpha particles as a big, slow bowling ball that rolls and stops quickly, beta particles as a fast baseball, and gamma rays as invisible X‑ray beams that can go right through you.
Half‑life – the ticking clock of decay
The term half‑life is the time needed for half of a sample of radioactive atoms to decay. Imagine you have 100 popcorn kernels popping in a microwave. If after 30 seconds only 50 are left un‑popped, that 30‑second span is the half‑life for that batch.
Worked example: A sample of iodine‑131 has a half‑life of 8 days. How much will remain after 24 days?
24 days = 3 half‑lives (8 days × 3). After each half‑life the amount halves:
- After 1st half‑life: 50 % left
- After 2nd half‑life: 25 % left
- After 3rd half‑life: 12.5 % left
So only 12.5 % of the original iodine‑131 is still radioactive after 24 days.
Radioactive decay series
Many unstable nuclei don’t become stable in one step. They go through a chain of decays – a decay series. Below is a simple flowchart that shows a typical series from a heavy parent nucleus to a stable daughter.
Safety and everyday uses
Even though radiation can be harmful, we harness it every day. Medical X‑rays, cancer‑fighting radiotherapy, and smoke detectors all rely on controlled radioactive sources. The key is proper shielding – using the right material to block unwanted radiation.
Quick recap
- Radioactivity = unstable nucleus releasing radiation to become stable.
- Alpha = heavy, +2 charge, stopped by paper.
- Beta = light, ±1 charge, stopped by thin metal.
- Gamma = pure energy, needs dense shielding.
- Half‑life tells how fast a sample decays.
- Decay series often involve several steps before reaching stability.
📝 Likely Exam Questions
1. Define half‑life and calculate the remaining mass of a 20 g sample of a radionuclide with a half‑life of 5 days after 15 days.
Answer: Half‑life is the time for half the nuclei to decay. After 15 days (3 half‑lives) the mass = 20 g × (½)³ = 20 g × 1/8 = 2.5 g.
2. List the three types of radiation and give one practical use for each.
Answer: Alpha – smoke detectors (alpha source ionises air). Beta – medical imaging (beta emitters in PET scans). Gamma – sterilising medical equipment (gamma rays kill bacteria).
3. Explain why a thick lead sheet is required to protect against gamma radiation but not against alpha particles.
Answer: Gamma rays are high‑energy photons with no mass or charge, so they penetrate deeply; dense materials like lead absorb them. Alpha particles are heavy, charged, and lose energy quickly, stopping in a few centimetres of air or a sheet of paper.
4. A sample of uranium‑238 undergoes alpha decay to become thorium‑234. Write the nuclear equation for this step.
Answer: 238U → 4He + 234Th.
5. Describe one safety precaution you should take when handling a radioactive source in a laboratory.
Answer: Always use appropriate shielding (e.g., lead glass or acrylic) and keep a safe distance, following the principle of “time, distance, and shielding” to minimise exposure.