When an atom’s nucleus gives off a particle/radiation and becomes stable, it’s done. It is no longer radioactive. Therefore, a mass of radioactive material (isotope) will gradually decline in radioactivity. This gradual decline in radioactivity is known as radioactive decay.
How fast is radioactive decay? This is measured it terms of half-life. Half-life is the length of time it takes/will take for the radioactivity of a given isotope to decline to one half of its starting level. As described in the following video, the starting amount doesn’t matter. The time it takes for the given isotopeto decay to half its starting amount remains the same.
The half-lives of different isotopes varies from fractions of a second to billions of years. To satisfy kid’s curiosity, the half-life of common isotopes is give in the following table: