An isotope is radioactive when its nucleus is unstable, so it spontaneously changes into a more stable nucleus over time and releases radiation in the process.

Why it happens

The main cause is an imbalance in the nucleus, usually in the number of protons and neutrons.

If the nucleus has too many neutrons, too many protons, or simply too much total nuclear energy, the strong nuclear force can no longer hold it in a stable arrangement, so it decays.

What “radioactive” means

A radioactive isotope, or radioisotope, is one whose nucleus has a chance of spontaneously changing into another nucleus over time.

That change may produce alpha particles, beta particles, neutrons, or photons, and the decay can happen in a chain until a stable nucleus is reached.

Simple example

Uranium-238 is radioactive because its nucleus is very large and unstable, while uranium-235 is also radioactive but differs by three neutrons.

In general, heavier nuclei need a different proton-to-neutron balance to stay stable, which is why many large atoms are radioactive.

In one line

An isotope is radioactive because its nucleus is not in a stable energy arrangement, so it naturally decays toward a more stable state.