The result is a gain in 1 in atomic number and no change in mass. An example is the decay of rubidium to strontium. The result is a loss of 1 in atomic number and no change in mass. An example is the decay of potassium to argon. The usefulness of radioactive decay to geology comes from the fact that the rate of decay is a constant that is unaffected by any physical process.
Figure shows how the decay of parent atoms and growth of daughter atoms with time.
Magma is generated and begins to cool. Radioactive decay occurs throughout this process, but since the magma is a liquid the parent atoms quickly separate from the daughter atoms. When the rock solidifies that separation can no longer occur and the newly formed daughter atoms are trapped. Scientists sample the rock and in the lab release both the parent and daughter atoms, counting each. The decay rates have been experimentally determined and from the decay equation the age of the rock can measured.