GED Science Practice Test: Radioactivity, Isotopes And Radiocarbon Dating

We have focused a great deal on electrons, particularly valence electrons, in this unit, because they are so important to understanding many of the physical and chemical properties of elements and compounds.  We now turn our attention to neutrons, and how variances in the number of neutrons in an atom explains radioactivity, which is an important property of some materials.

Isotopes: The number of neutrons in the nucleus of an atom of a specific element can vary. For example, carbon, which normally has 6 protons and 6 neutrons, also has versions that have 7 neutrons or 8 neutrons in the nucleus. These kinds of atoms, that have atypical numbers of neutrons in the nucleus, are known as isotopes.  Because different isotopes of an element have different numbers of neutrons, different isotopes of an element also have different atomic masses.  The following table shows the three common isotopes of carbon:

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There is a great deal of other information in the diagram above.  Isotopes are named with a combination of their element symbol and their mass (e.g., carbon-12, carbon-13, etc.).  Some isotopes are more common than other isotopes; carbon-12 makes up ~99% of the carbon on earth, while carbon-13 makes up ~1% of the carbon on earth.  If you look up the atomic mass of carbon in a periodic table, you will notice that it is not a whole number, despite the fact that the atomic mass represents the number of protons and neutrons (which, themselves, can only be whole numbers).  The mass of carbon in the periodic table is 12.0107.  The reason for this is that the atomic mass in the periodic table is an average of the atomic masses of the isotopes of carbon.  The atomic mass of carbon (12.0107) is close to 12, because most of the carbon on earth (99%) is carbon-12.  However, the small amount of carbon-13 and carbon-14 raise the average atomic mass just a bit.

Isotopes of an element differ in their stability.  Carbon-12, for example, is stable, while carbon-14 is not.  This instability leads an atom to give off particles or energy in order to become more stable.  The emission of these particles or energy is called radioactivity.  The particles or energy, themselves are called radiation.  The following diagram shows the different types of radiation and their ability to penetrate certain substances.

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Taken from: http://www.daviddarling.info/encyclopedia/G/gamma_rays.html

As you can see in the diagram, some type of radiation can penetrate through human body tissues.  Even alpha particles, which can be stopped by a sheet of paper, can interact with human skin.  Radiation can lead to changes within the DNA of our cells, causing mutations, and possibly cancer.  However, radiation is not all bad.  Radiation is how x-rays and some cancer therapies work.

The radioactive decay of radioactive isotopes can be used to establish an age of fossils, rocks, and other substances.  The radioactive decay of uranium, for example, can be used to determine an age for rocks.  The radioactive decay of carbon can be used to date fossils.  The reason that the radioactive decay of isotopes can be used to date items is because radioactive decay occurs at very predictable rates.  Different rates use different units.  For example, if you talk about the speed of a car, you refer to its speed in miles per hour.  In radioactive decay, speeds are talked about in terms of half-lives.  A half-life is the amount of time it takes for half of a radioactive substance to decay.  For example, carbon-14 decays be emitting a beta particle to become nitrogen.  The half-life of carbon-14 is 5730 years.  In other words, after 5730 years, half of the initial radioactive carbon in a sample will be left.  The following graph shows the radioactive decay of carbon as it goes through its half-lives:

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So for example, when an object is 5730 years old, it will have half of the amount of carbon-14 it originally had (the rest will have decayed by emitting a beta particle and becoming a more stable element).  After 11,460 years, another half of the carbon will decay, leaving ¼ the radioactive carbon that was originally in the object.

Given this information, it may be difficult to understand how carbon dating works.  After all, you need to know the amount of radioactive carbon-14 the object originally had in order to figure out how much that carbon-14 has decayed.  Fortunately, we do already know that carbon-14 makes up less than 1% of the carbon on earth.  Thus, we would expect that any object would be made up of only 1% carbon-14.  If we find an object that only has 0.5% carbon-14, half of what it is expected to have, then we can estimate its age to be 5730 years old.

There are limits to radiocarbon dating.  One limitation is that carbon cannot date objects much older than about 50,000 years old.  This excludes almost all rocks from being dated using radiocarbon dating, because they are much older than 50,000 years old.  Generally, nine half-lives is the upper limit for accurate radioactive dating.  After nine half-lives, the amount of radioactive material left is so small as to be practically impossible to measure.  Below is a table of some isotopes used for radioactive dating, their half-lives, and the type of radiation emitted:

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