2. Aim Aim: How do geologists determine the absolute age of a rock? Radioactive Dating I. Radioactive Dating Absolute Age A. Absolute Age – exact age in years Isotope 1. Isotope – same element that has different number of neutrons in nucleus. Ex: Carbon12 (C 12 ) and Carbon 14 (C 14 )

3. 2. Radioactive isotope isotope – when the atom has fewer or more neutrons in the nucleus. A radioactive isotope will break down naturally into a lighter element called a decay product product. 3. Decay product product – the substance produced after a radioactive isotope breaks down. The decay product is stable and will not turn into anything else.

4. Radioactive Isotope Decay ProductHalf Life (years) C 14 N 14 5.7 x 10 3 K 40 Ar 40 or Ca 40 1.3 x 10 9 U 238 Pb 206 4.5 x 10 9 Rb 87 Sr 87 4.9 x 10 10

5. Half Life B. Half Life – the time taken for the activity of decay to reduce the total amount of radioactive substance in a material to half its original amount. 1. If you know the original content of a radioactive material and compare it to the present content of the same radioactive material, you can then determine the age of the material.

6. 2. The half-lives of radioactive isotopes are different for different substances. a. Some substances have short half-lives (ex: C 14 ) and are good for dating recent organic remains (1,000 to 50,000 years)

7. C 14 100% Radioactive 1 Half Life 2 Half Lives 3 Half Lives 4 ½ Lives

8. 100% Radioactive C 14 C 14 1 Half Life 5,700 years 2 Half Lives 11,400 years 3 Half Lives 17,100 years 22,800 years C 14 N 14 N 14 4 ½ Lives

9. http://www.colorado.edu/physics/2000/isotopes/lifetime.html http://vcourseware5.calstatela.edu/VirtualDating/files/2.0_HalfLife.h tml