Radioactive decay
The speed at which this change happens, is different for each element. Radioactive decay is governed by chance: The time it takes, on average for half the atoms of a substance to change is named the half-life. The rate is given by an exponential function. As an example, iodine (131I) has a half-life of about 8 days. That of  plutonium ranges between 4 hours (243Pu) and 80 million years (244Pu)