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Unstable atoms break down at random. How can we still say when half of them will be gone?
Radioactive Decay lab · Last updated: 16 October 2026
You watch a crowd of unstable atoms, each breaking down at a completely random moment. No one can say when any single atom will go, yet the crowd as a whole follows a precise pattern. They find out what a half-life is and how it lets scientists date ancient bones and rocks.
What you'll discover
- Each unstable atom breaks down at a random moment that cannot be predicted.
- With huge numbers of atoms, the randomness averages out, so half are always gone after the same time.
- After one half-life half the atoms remain, after two a quarter, after three an eighth, and so on.
The science in plain words
Some atoms have nuclei that are unstable. Sooner or later such a nucleus breaks down, giving out radiation and turning into a different nucleus. This is radioactive decay. The radiation may be an alpha particle, a beta particle or gamma rays. Nothing we do to the atom, such as heating or squeezing it, changes when this happens.
For a single atom, decay is completely random, like flipping a coin each second and waiting for heads. You cannot predict when it will happen. But a small sample contains billions upon billions of atoms. Each one has the same chance of decaying in any given second, so the overall pattern becomes very predictable, just as a huge number of coin flips gives almost exactly half heads. The time it takes for half of the atoms in a sample to decay is called the half-life.
Half-lives range from tiny fractions of a second to billions of years, but for any particular isotope it is fixed. After one half-life, half the atoms remain; after two, a quarter; after three, an eighth. Carbon-14 has a half-life of about 5,730 years. Living things take in carbon-14 while alive, and after death it decays, so measuring how much is left lets scientists work out the age of bones, wood and cloth up to about 50,000 years old.
Where it fits
- India, NCERT: Goes beyond the NCERT syllabus for classes 6 to 10; it builds on Class 9 Structure of the Atom.
- United States: Grades 9 to 12
- Next Generation Science Standards: HS-PS1-8
Common questions
What is a half-life?
A half-life is the time it takes for half of the radioactive atoms in a sample to decay. It is fixed for each isotope. After one half-life half the atoms remain, after two half-lives a quarter remain, and the amount keeps halving at the same steady rate.
If decay is random, how can we predict it?
Each individual atom decays at a random moment, but every atom has the same chance of decaying in a given time. A sample contains so many atoms that the random events average out, just as tossing a million coins gives almost exactly half heads.
How does carbon dating work?
Living things take in carbon, including a small amount of radioactive carbon-14. After death no new carbon-14 is taken in, and what is there decays with a half-life of about 5,730 years. Measuring how much remains tells scientists roughly how long ago the organism died.
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- Star Life Cycle: Heavy stars have more fuel. So why do they die young?
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Try it yourself
The Radioactive Decay lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.