Atoms with certain characteristics are radioactive. which of those below will always be radioactive?

All atoms with an atomic number over 83

Explanation:

The electromagnetic repulsion of the protons in the nucleus is overcome by strong nuclear forces that keep the nuclear particles together. However, as the atomic number gets bigger so does the size of the nucleus. Therefore, the repulsive electromagnetic forces become greater than the nuclear forces hence the atom becomes unstable. Instability of the nucleus means the atoms become naturally radioactive.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.