Radiometric Dating t the percentage of radioactive atomsthat decay dur that Notice ing one half-life is always the same: 50 percent. However, the actual number of atoms that decay with the passing of each half ife continually decreases. Thus, as the percentage of radioactive parent at declines, the proportion of stable daughter atoms rises, with the increase in daughter atoms just matching the drop in parent atoms. This fact is the key to radio- metric dating.