Radioactive Decay
Unstable nuclei emit radiation to become stable.
About this topic
Unstable nuclei decay spontaneously to become more stable by emitting radiation. Alpha decay emits a helium nucleus (⁴₂He), reducing the mass number by 4 and atomic number by 2. Beta decay converts a neutron into a proton and emits an electron, raising atomic number by 1. Gamma rays are high-energy photons that carry away excess energy.
Nuclei become unstable when the balance of protons and neutrons is wrong, or when they are simply too large. Decay is random for any one atom, but for a large sample it follows a predictable pattern described by half-life, the time for half of the nuclei to decay. After one half-life 50 percent remain, after two 25 percent, after three 12.5 percent. Alpha particles are stopped by paper, beta by a few millimetres of aluminium, and gamma needs thick lead or concrete.
Example: iodine-131 has a half-life of about 8 days, so a 80 g sample falls to 40 g after 8 days and 20 g after 16 days. Uses include smoke detectors, cancer therapy, medical tracers and dating rocks, but exposure can damage cells, so shielding, distance and short exposure time are the safety rules.
A misconception is that radioactive material becomes zero at twice the half-life; it only keeps halving. Also, radiation is not the same as contamination, and decay rate is unaffected by temperature or chemical state.
Key ideas
- Alpha, beta and gamma
- Half-life describes decay rate
- Used in dating and medicine
Quick check
Use cases: where Radioactive Decay helps
- Homework: review the key idea behind Radioactive Decay before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Radioactive Decay without notes.
- Visual learning: play with the interactive lab to see how Radioactive Decay changes when you adjust its inputs.
- Teaching a friend: explain Radioactive Decay in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Radioactive Decay?
Unstable nuclei emit radiation to become stable.
What will I learn in Radioactive Decay?
Alpha, beta and gamma. Half-life describes decay rate. Used in dating and medicine.
Is Radioactive Decay part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Radioactive Decay lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Radioactive Decay?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.