Unstable nuclei

1. Most of the nuclei around us are stable, which means they stay the same forever.
2. Some nuclei are unstable, usually because they have too many neutrons, too many protons, or too many nucleons altogether.
3. An unstable nucleus becomes more stable by giving out nuclear radiation, and this process is called radioactive decay.
4. An isotope with unstable nuclei is called a radioactive isotope, so carbon-14 is radioactive while carbon-12 is stable.
5. Radioactive decay is spontaneous, which means it happens by itself and cannot be started, stopped or sped up by heating, cooling, applying pressure or changing chemical conditions.
6. An unstable nucleus can emit four kinds of radiation: alpha particles, beta particles, gamma rays and neutrons.

Alpha particles

1. An alpha particle consists of two protons and two neutrons bound tightly together.
2. This is exactly the same as the nucleus of a helium atom, but with no electrons around it.
3. An alpha particle has a relative mass of $4$ and a relative charge of $+2$, which makes it the heaviest and most highly charged of the emissions.
4. Alpha particles are usually emitted by large, heavy nuclei, such as uranium-238, radium-226 and americium-241.

Beta particles

1. A beta particle is a high-speed electron.
2. The beta particle is created inside the nucleus at the moment of decay and is then ejected, so it is not one of the electrons that were already orbiting the nucleus.
3. A beta particle has a relative charge of $-1$ and a very small mass, about $\frac{1}{2000}$ of the mass of a proton.
4. Beta particles are usually emitted by nuclei that have too many neutrons, such as carbon-14 and strontium-90.

Gamma rays

1. A gamma ray is not a particle of matter but a burst of electromagnetic radiation with a very high frequency.
2. Gamma rays have no mass and no charge, and they travel at the speed of light.
3. Gamma rays are emitted when a nucleus is left with too much energy, which often happens straight after it has emitted an alpha or beta particle.
4. Cobalt-60 and technetium-99m are examples of isotopes whose nuclei emit gamma rays.

Neutrons

1. Some unstable nuclei with a large excess of neutrons can emit a single neutron.
2. An emitted neutron has a relative mass of $1$ and no charge.
3. Neutron emission is much less common than alpha, beta or gamma emission in naturally radioactive materials.
4. Because a neutron has no charge, a free neutron passes easily through many materials.