Unit II: Assumptions of Radiometric Dating

Read Seeds: Section 5.2

Lecture: Radiometric dating is based on several premises. Scientists are frequently involved in determining the nature of a universe that they cannot observe directly. It may be that the object of study may be not directly observable because of its great distance, its small size or having occurred at some time in the distant past. In order for scientist to draw conclusions about events he can not observe directly, he must make some initial assumptions. He is much like a detective who comes upon the scene of a crime "after" the crime has occurred. Since he has not observed the crime directly, he must make some initial assumptions in order to deduce how and when the crime was committed . If any of the assumptions below is false, radiometric dating will produce erroneous results.

The basic assumptions made in radiometric dating are:

• Every radioactive element will decay at a constant rate. The rate at which each element decays is its half-life (def.)

• The rate of decay is specific to a particular radioactive element (see list of half lifes of various radioactive elements).

• When the substance containing a radioactive parent was first formed, there was not daughter element present. It is assumed that the daughter is derived solely from the decay of the radioactive parent. If daughter atoms were present that were not the result of the decay process the calculated date would be unreliable.

• From the time when the substance containing radioactive elements first came into existence until the time that the material was analyzed and dated, the system had been closed; in other words there had been in infusion or removal of either parent or daughter atoms.

• All daughter atoms contained within the radioactive substance were created by the radioactive decay of the corresponding parent atom. This is a repeat of the previous assumption that the system is a closed system.

Complete these Self-check Questions:

23. Why is it important that we assume a closed system while dating materials by the radiometric dating method. (ans.)

Homework Questions: (To submit your answers to the homework questions, first copy the questions from this page and paste them into the homework form. Insert your answers below the questions. Fill in the remaining form elements and submit. Your homework will be e-mailed to me. I will return your graded homework to you in the private e-mail of the comm center).

There are none for this lecture.

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• Radioactivity