Radioactive elements are elements made up of atoms with unstable nuclei. Unstable nuclei emit ionising radiation in the form of alpha particles, beta particles or gamma rays in a process called radioactive decay. Polonium is thought of as one of the most radioactive naturally occurring elements, due to the high intensity of alpha particles it emits.
Just one gram of polonium can self heat to temperatures of 500 degrees C. However, there are elements with a higher radioactivity that have been synthesised in a lab, such as ununpentium, which is so unstable it can only last a few fractions of a second before decaying.
Answered by Rik Sargent