Unstable nucleus
A nucleus with too many or too few neutrons relative to the number of protons, causing it to decay radioactively in an attempt to reach a stable configuration. Instability arises from an unfavourable neutron-to-proton ratio and insufficient nuclear binding energy to hold the nucleus together permanently. Unstable nuclei emit alpha particles, beta particles, or gamma rays during decay.
Real World
Technetium-99m is an unstable isotope with a 6-hour half-life, deliberately chosen by hospitals like Great Ormond Street for medical imaging because it decays quickly enough to limit patient radiation exposure.
Exam Focus
State the specific decay mode (alpha, beta-minus, beta-plus, or gamma) — simply writing 'radioactive decay' scores zero.
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