Grade 11 · Physics
Radioactivity, Isotopes & Half-Life
Why it matters
Radioactive decay and dating.
Last reviewed 2026-09-03 · accuracy standard
What is radioactive decay and half-life?
Short answer
Radioactive decay is the spontaneous breakdown of an unstable atomic nucleus, releasing particles or energy as alpha, beta, or gamma radiation. Half-life is the time it takes for half of a radioactive sample to decay. After each half-life the remaining amount halves, so decay follows a predictable exponential pattern used in radiometric dating.
Concept preview
What you will be able to do
- 01Describe the nucleus, isotopes, and nuclear notation
- 02Distinguish alpha, beta and gamma radiation and write decay equations
- 03Use the concept of half-life
At the heart of every atom is a tiny, dense nucleus packed with protons (positive) and neutrons (neutral) — together called nucleons. The number of protons, the atomic number , defines the element; the total number of nucleons is the mass number . So , where is the neutron count.
School Plan
Radioactivity, Isotopes & Half-Life
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Related topics
Frequently asked questions
- What are the three types of radiation?
- Alpha (a helium nucleus), beta (an electron), and gamma (high-energy electromagnetic radiation).
- What does half-life tell you?
- The time for half of a radioactive sample to decay; it is constant for a given isotope regardless of sample size.