Explain fully why some nuclei are radioactive.

An atom is stable if the forces among the particles that makeup the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced; if the nucleus has an excess of internal energy. Instability of an atom's nucleus may result from an excess of either neutrons or protons.

Explanation:

why are some atoms radioactive?

the delicate balance of forces among particles keeps the nucleus stable. Any change in the number the arrangement or the energy of the nucleons can upset this balance and cause the nucleus to become unstable and create a radioactive atom

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.