Isotopes have the same chemical properties, but often have slightly different physical properties, due to the differences in their mass. Many naturally occurring isotopes are stable, which means that their nuclei remain the same and do not change spontaneously. However, in some cases, a particular isotope, or several isotopes even, of an element are unstable. Unstable isotopes want to become stable. In some cases, a particular unstable isotope will decay into another unstable isotope, which will itself then decay, and so on, until a stable isotope is finally reached.
Decay chains can be very long or very short, depending on the particular isotope.
It takes energy to hold the protons and neutrons together in the nucleus. There are several different forms of ionising radiation. Alpha and beta radiation are tiny atomic particles travelling at high speeds. Radiation can cause damage to living things, like the cells in our body, but this can also be a good thing. For example, radiotherapy treatment for cancer works by using radiation to destroy the cancer cells.