What is Radiation? Properties of Radioactive Isotopes
Half-Life and Radioisotopic Dating - Chemistry LibreTexts
Carbon has a large number of stable isotopes. All carbon atoms contain six protons and six electrons, but the different isotopes have different numbers of neutrons. The amount of carbon in the atmosphere has not changed in thousands of years. Even though it decays into nitrogen, new carbon is always being formed when cosmic rays hit atoms high in the atmosphere. Plants absorb carbon dioxide from the atmosphere and animals eat plants. This means all living things have radioactive carbon in them.
Radioactive isotopes have a variety of applications. Generally, however, they are useful because either we can detect their radioactivity or we can use the energy they release. Radioactive isotopes are effective tracers because their radioactivity is easy to detect.
A radioactive tracer , radiotracer , or radioactive label , is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling or radiotracing is thus the radioactive form of isotopic labeling. Radioisotopes of hydrogen , carbon , phosphorus , sulfur , and iodine have been used extensively to trace the path of biochemical reactions. A radioactive tracer can also be used to track the distribution of a substance within a natural system such as a cell or tissue ,  or as a flow tracer to track fluid flow.