Some nuclei are unstable and spontaneously change into other nuclei. This process is called radioactive decay.

In alpha decay, a nucleus emits an alpha particle, which is a helium nucleus with $2$ protons and $2$ neutrons.

In beta-minus decay, a neutron changes into a proton and emits an electron and an antineutrino. In beta-plus decay, a proton changes into a neutron and emits a positron and a neutrino.

Gamma decay happens when a nucleus drops from a higher-energy state to a lower-energy state and emits a gamma ray, which is very high-energy light.