Why are some chemical elements radioactive? How are radioactive elements created?

Elements (atoms) are radioactive (actively discharge particles in straight lines along radii!) because their nuclei contain too many neutrons or binding energy. The atoms may spontaneously eject an electron, neutron, proton or alpha particle and excess energy as gamma rays in a decay series until stable. Uranium can decay until it becomes lead, etc. When a supernova explodes it fuses matter into very heavy elements and some may be so heavy and unstable that they all decay within a fraction of a second while others may have half lives of billions of years.