Question

why are isotopes with a neutron:proton ratio far from the band of stability radioactive?
they have too many electrons
they have an unstable nucleus
they have too much energy
they have a stable nucleus

Explanation:

Radioactive isotopes have unstable nuclei. The Band of Stability represents the neutron - proton ratios where nuclei are stable. Isotopes far from this band have nuclei that are energetically unfavorable and will undergo radioactive decay to achieve a more stable state. The number of electrons is not directly related to radioactivity (as radioactivity is a nuclear property). Having too much energy is a consequence of an unstable nucleus rather than the root cause. A stable nucleus would not be radioactive.

Answer:

They have an unstable nucleus.