About this simulation
Most combinations of protons and neutrons fall apart. About 250 are stable; over 3,000 more are radioactive, and each nucleus decays at its own random moment.
What you're seeing
- Nuclei: protons red, neutrons grey. The proton number sets the element; protons plus neutrons give the mass number (carbon-14).
- The ring around an unstable nucleus is its clock: how long it has existed, on a log scale shared by every nucleus, so billions of years pass in seconds. The notch marks its half-life on the same scale, so short-lived nuclei have their notch early.
- The chart plots neutrons N (up) against protons Z (across), coloured by how each nuclide decays. It zooms in on the selected nucleus, with arrows for each decay; its equations appear underneath.
Key idea
The strong force holds nucleons together; the protons repel each other. Light stable nuclei have N ≈ Z; heavier ones need extra neutrons, so the valley of stability bends towards N ≈ 1.5Z. Too many neutrons: β− decay (n → p + e− + ν̄). Too many protons: β+ decay or electron capture. Too heavy: α decay or fission. Half of a large sample decays in one half-life, but when a single nucleus decays is pure chance.
Things to try
- Drop a neutron onto uranium-235: it usually splits in two, freeing two or three neutrons. The U-235 chain reaction setup shows where that leads, counting the fissions in each generation of neutrons: a count stays grey while it can still grow.
- Carbon dating runs one steady clock for a whole sample: about half is left after each half-life.
- Add a neutron to uranium-238 and watch it become plutonium-239, or flick it through a crowd of light nuclei.
How to use it
Drag from the tray (or tap) to add; drop onto a nucleus to combine, or flick to throw. Drag onto the bin or off the canvas to remove. folds the tray away. Tap the chart to enlarge it. opens the settings (speed, setups, any nuclide), goes fullscreen, clears and resets.
Data: IAEA ENSDF and NUBASE2020; other nuclides are estimated with the semi-empirical mass formula.