24.2. Half-Life http://www.ck12.org
Practice Problems- The half-life of polonium-218 is 3.0 min. How much of a 0.540 mg sample would remain after 9.0 minutes
have passed? - The half-life of hydrogen-3, commonly known as tritium, is 12.26 years. If 4.48 mg of tritium has decayed to
0.280 mg, how much time has passed?
Radioactive Dating
Radioactive datingis a process by which the approximate age of an object is determined through the use of certain
radioactive nuclides. For example, carbon-14 has a half-life of 5,730 years and is used to measure the age of organic
material. The ratio of carbon-14 to carbon-12 in living things remains constant while the organism is alive because
fresh carbon-14 is entering the organism whenever it consumes nutrients. When the organism dies, this consumption
stops, and no new carbon-14 is added to the organism. As time goes by, the ratio of carbon-14 to carbon-12 in the
organism gradually declines, because carbon-14 radioactively decays while carbon-12 is stable. Analysis of this
ratio allows archaeologists to estimate the age of organisms that were alive many thousands of years ago. The ages
of many rocks and minerals are far greater than the ages of fossils. Uranium-containing minerals that have been
analyzed in a similar way have allowed scientists to determine that the Earth is over 4 billion years old.
Decay Series
In many instances, the decay of an unstable radioactive nuclide simply produces another radioactive nuclide. It may
take several successive steps to reach a nuclide that is stable. Adecay seriesis a sequence of successive radioactive
decays that proceeds until a stable nuclide is reached. The terms reactant and product are generally not used for
nuclear reactions. Instead, the terms parent nuclide and daughter nuclide are used to refer to the starting and ending
isotopes in a decay process. The figure below (Figure24.7) shows the decay series for uranium-238.
FIGURE 24.7
The decay of uranium-238 proceeds
along many steps until a stable nuclide,
lead-206, is reached. Each decay has its
own characteristic half-life.