Radioactivity is the property of unstable atoms (radionuclides) of spontaneously emitting ionising radiation. Their instability is due to the particular ratio of neutrons to protons in their core (nucleus). They emit ionising radiation as they attempt to transform (‘decay’) into a more stable form. Some radionuclides decay to a stable atom (nuclide); others decay to another radionuclide, which then also emits ionising radiation as it decays.