All subsequent elements are radioactive and therefore unstable ... Estimates suggest that there is only about 30 g of francium in the whole of the Earth's crust. In the 1930s it was the turn ...

Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,730 years, but the half-life of francium-223 is just 20 minutes. Graph of count rate against ...