Notice that the percentage of radioactive atoms that decay during one half life is always the same: 50 percent. However, the actual number of atoms that decay with the passing of each half-life continually decreases Thus, as the percentage of radioactive parent atoms declines, the proportion of stable daughter atoms rises, with the increase in daughter atoms just matching the drop in parent atoms. This fact is the key to radiometric dating. Of the many radioactive isotopes that exist in nature, five have proved particularly useful in providing radiometric ages for ancient rocks (Table 11.1). Rubidium-87, thorium-232, and the two isotopes of uranium are used only for dating rocks that are millions of years old, but potassium-40 is more versatile. Although the half life of potassium-40 is 1.3 billion years, analytical techniques make possible the detection of tiny amounts of its stable daughter product, argon-40, in some rocks that are younger than 100,000 years Lt is important to realize that an accurate radiometric date can It is important to realize that an be obtained only the mineral remained a closed system during the entire period since its formation. correct date is possible unless there was neither the addition nor loss of parent or daughter isotopes. This is not always the case. In fact, an important limitation of the potassium – argon method arises from the fact that argon is a gas, and it may leak from minerals, throwing measurements. Cross-checking of samples, using two different radiometric methods, is done where possible to ensure accurate age determinations.