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 )

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.

Radioactive Isotope Decay ProductHalf Life (years) C 14 N x 10 3 K 40 Ar 40 or Ca x 10 9 U 238 Pb x 10 9 Rb 87 Sr x 10 10

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.

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)

C % Radioactive 1 Half Life 2 Half Lives 3 Half Lives 4 ½ Lives

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

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