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Why are radioactive isotopes a valuable tool in molecular biology? Molecular biology mostly takes place on a scale too small to be directly observed by researchers. While cells, organelles, and even macromolecules can be visualized with high-powered microscopes, the molecular processes that drive biology cannot be seen and need to be inferred from experiments. Radioactive isotopes are versions of elements such as carbon, nitrogen, or phosphorus that unstable and release neutrons to become more stable isotopes. The radioactive signal given off by the emission of neutrons allows scientists to detect the isotopes even when they cannot be seen. Adding isotopes to biological systems allows scientists to trace both molecular pathways and the locations at which these pathways occur.