Nuclear radiation is emitted by those elements whose:

Nuclear radiation is released by elements whose nuclei are unstable. An unstable nucleus means the balance of protons and neutrons is not right, causing the atom to break down and release energy in the form of radiation. This process is called radioactive decay. Stable nuclei do not emit radiation. The stability or instability is related to the nucleus, not the molecule. So, the correct answer is nuclei are unstable.