1.1 Atoms, isotopes, ions, and molecules: the building blocks  (Page 3/61)

Evolution connection

Carbon dating

Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon-14 ( ¹⁴ C) is a naturally occurring radioisotope that is created in the atmosphere from atmospheric ¹⁴ N (nitrogen) by the addition of a neutron and the loss of a proton because of cosmic rays. This is a continuous process, so more ¹⁴ C is always being created. As a living organism incorporates ¹⁴ C initially as carbon dioxide fixed in the process of photosynthesis, the relative amount of ¹⁴ C in its body is equal to the concentration of ¹⁴ C in the atmosphere. When an organism dies, it is no longer ingesting ¹⁴ C, so the ratio between ¹⁴ C and ¹² C will decline as ¹⁴ C decays gradually to ¹⁴ N by a process called beta decay—the emission of electrons or positrons. This decay gives off energy in a slow process.

After approximately 5,730 years, half of the starting concentration of ¹⁴ C will have been converted back to ¹⁴ N. The time it takes for half of the original concentration of an isotope to decay back to its more stable form is called its half-life. Because the half-life of ¹⁴ C is long, it is used to date formerly living objects such as old bones or wood. Comparing the ratio of the ¹⁴ C concentration found in an object to the amount of ¹⁴ C detected in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material, such as the pygmy mammoth shown in [link] , can be calculated with accuracy if it is not much older than about 50,000 years. Other elements have isotopes with different half lives. For example, ⁴⁰ K (potassium-40) has a half-life of 1.25 billion years, and ²³⁵ U (Uranium 235) has a half-life of about 700 million years. Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms to understand how organisms have evolved from earlier species.