If you check mendeleevs periodicv table, you will find the vast majority of elements heavier than led as radioactive. Scientists say that this is so because as the nucleus gets larger, the short range nuclear binding energy is unable to hold the nucleus. so, alpha particles start falling apart from the nucleus with emmisions of gamma rays. It is but obvious that when earth would have been molten, all such heavy elements would have settled at the core. That explains the tremendous heat inside earth.
