21. Why are radioactive nuclei unstable? a. There is not the proper neutron to proton ratio resulting in a stable nucleus
b. There is too much energy stored in the bonds between atoms
C. They need to gain/lose electrons to have a full valence energy level
d. Protons reverse their charges and begin to repel one another

a. There is not the proper neutron to proton ratio resulting in a stable nucleus

Explanation:

Radioactive nuclei are unstable because they do not have a proper neutron to proton ration that can result in a stable nucleus for them.

Every atomic nucleus has specific neutron to proton ratio. The ratio ensures the stability of the nucleus. Any nucleus with a neutron/proton combination different from its stability ratio will be unstable and split into one or more other nuclei. This is usually with an attendant emission of small particles of matter and electromagnetic ionizing radiation and energy.