Unit II: The Age of the Solar System

Read Seeds: Section 5.2

Figure 1: Figure 2:

Lecture: The solar nebula hypothesis provides a mechanism for solar system formation. How does one determine the date for its formation? Since we assume that the bodies that make up the solar system were formed at essentially the same time, we can determine the age of solar system by dating the bodies that make it up. One method that has been used is radiometric dating.

Some elements are radioactive. Radioactive elements are the elements that are found at the bottom of the periodic table. The forces which normally hold the protons and neutrons together in the nucleus are insufficient, making these nuclei naturally unstable. Spontaneously the nuclei release nuclear particles in a process called radioactive decay (def.) (figure 1). The result of radioactive decay is the release of both nuclear particles and energy. The original radioactive atom is called the parent (def.). The decay process changes the atomic number (def.). of the parent, thereby its changing its elemental identity. The new element that is produced is called the daughter (def.)(figure 2). The daughter may or may not be radioactive. Eventually, however, the decay process will continue until a stable, non-radioactive daughter is formed.

The relative abundance of the parent and daughter atoms is based on the theory of probability. Think of a coin. A coin when tossed can turn up either as a heads or a tails. If one flips 100 coins, the probability would be great that 50 of them would come up heads and 50 of them would come up tails. Likewise if we were to start out with a rock that at one time contained 100 radioactive atoms, at one point in time 50 of them would have decayed to daughters atoms and 50 would still be parent atoms. The time that it takes for this proportion to b achieved is the half-life (def.) Every half-life the original abundance of parent atoms decreases by 50%, whereas the abundance of daughter atoms increases proportionally.

By knowing the half life of an element and the relative abundance of the parent and daughter, we can theoretically calculate the length of time the parent atoms in a substance have been decaying. This is referred to as radiometric dating. However, all scientists know that like most scientific theories, the principle of radiometric dating is based on several assumptions. The validity of these assumptions ultimately determine the reliability of the radiometric dating method.

17. One type of radioactive decay is "alpha decay" Alpha decay occurs when a parent atom ejects a helium nucleus (2 protons and 2 neutrons). Uranium 238 alpha decays. What element is produced? (ans.)
    

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• Radioactivity