What is radioactivity and what does it have to do with the age of the earth?

Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation. Radioactive elements decay at a predictable rate over time, which allows scientists to use radiometric dating to determine the age of rocks and minerals on Earth. By measuring the ratiOS of different isotopes in a sample, such as uranium-lead dating or carbon-14 dating, scientists can estimate the age of the Earth to be around 4.5 billion years.