Isotopes

Some elements can exist in several different forms. These forms are called isotopes. All isotopes of an element have the same atomic number. Remember that atomic number is the number of protons in the nucleus of the atom. What is the atomic number of isotopes carbon-12 and carbon-14?

If all isotopes of an element have the same number of protons, how do they differ? To answer that, look closely at the number of neutrons (shown in purple) in the atoms above. How many neutrons does an atom of carbon-12 have? How many neutrons does an atom of carbon-14 have?

An atom's mass number is the sum of the number of protons and neutrons in the nucleus. The mass number of carbon-12 is 6 + 6, or 12. What is the mass number of carbon-14?

Half-Life

Some isotopes are unstable. The nucleus of an unstable atom breaks down in a process called radioactive decay. As it decays, the unstable atom releases fast-moving particles and energy. The time it takes for half of all the atoms in a sample to decay can be measured. This is called an isotope's half-life. Half-life is different for each isotope.

Carbon-14 is a radioactive isotope with a half-life of about 5,730 years. In other words, after 5,730 years have passed, one half of the original number of carbon-14 atoms in a sample will remain. Knowing the half-life of carbon-14 has helped scientists date plants and animals that lived up to 50,000 years ago. Scientists measure the amount of carbon-14 remaining in a sample of an object and then calculate how many half-lives have passed in the object's history. This process is called radioactive dating.

Other radioisotopes are used to make the dating of biological and geological samples as accurate as possible. Some of the radioisotopes commonly used in radioactive dating include

Uranium-235 (half-life = 704 million years)
Uranium-238 (half-life = 4.5 billion years)
Thorium-232 (half-life = 14 billion years)
Rubidium-87 (half-life = 49 billion years)

Select one of these isotopes and use an internet search engine to find Web sites with information about it. Write a report about the isotope. Explain the process used to date a sample containing your chosen isotope. Include information about why a scientist would select that isotope to date a particular geologic or biological sample.

Further Exploration

Radioactive isotopes are used for many purposes besides radioactive dating. For example, they can help doctors see inside the body and help engineers pinpoint weak spots in metal pipes. To find out more about isotopes, radioactivity, carbon-14 dating, and related topics, visit How Nuclear Radiation Works.