What is meant by the half-life of a radioactive isotope?
A radioactive isotope decays overtime. This is due to its unstable nuclei. Hence, overtime, they break down and form a completely different type of atom.
Essentially, the time taken for the number of nuclei in the isotope to half. This number varies among isotopes too.
For example, the half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.