Content Benchmark P.12.A.8
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Students know most elements have two or more isotopes, some of which have practical applications. I/S

Understanding isotopes requires a fundamental knowledge of atomic structure. The entire mass of an atom is almost all contained in the nucleus, which is composed of two primary particles known as protons and neutrons. Protons are positively charged particles and neutrons have no charge. Protons and neutrons have approximately the same mass, which is almost 2000 times the mass of an electron (a subatomic particle located outside the nucleus).

To learn more about the atomic theory of matter, go to http://www.visionlearning.com/library/module_viewer.php?mid=50.

Protons are responsible for the nature and behavior of a particular chemical element and each element has its own characteristic number of protons, called the atomic number. It is not possible to change the number of protons in an element through chemical processes, but through nuclear reactions, protons can be added or removed from a nucleus changing an atom from one element to another.

More details about nuclear reactions are found at http://www.lbl.gov/abc/Basic.html.

Nuclear reactions can also change the number of neutrons in a nucleus. However, neutrons, which are critical to nuclear stability, do not impact the charge of the nucleus. When neutrons are added or removed from an atom’s nucleus, the mass of the atom is changed, but not its charge. Therefore, the atom retains is elemental properties. Atoms of different mass, but with the same number of protons in the nucleus are called isotopes.

Most elements have two or more isotopes. For example, the most common form of carbon typically has a nucleus with 6 protons and 6 neutrons. But occasionally, a carbon nucleus will contain 6 protons and 7 neutrons or 6 protons and 8 neutrons. These are known as carbon isotopes. To identify isotopes of elements, scientists often name them by adding the numbers of protons and neutrons together. For example, in our carbon atom with 6 protons and 6 neutrons, this isotope would be called carbon-12. Its other isotopes are carbon-13 and carbon-14 respectively.

To learn more about isotopes, go to http://www.colorado.edu/physics/2000/isotopes/index.html.

In some isotopes, the atomic nuclei are unstable due to size of the nucleus and also the ratio of neutrons to protons. In an effort to achieve a more stable state, these nuclei emit particles and energy. Such activity is known as radioactivity and the process of emission, radioactive decay. Radioactive decay can occur in a very short span of time (e.g., microseconds) or over very long geologic time spans.

To get more details about radioactivity, go to
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html.

There are many beneficial applications of radioactive isotopes that are used in everyday activities. For example, cobalt-60, an isotope of cobalt that has one additional neutron in its nucleus compared stable cobalt-59, has applications in medicine as well as in food preservation. Due to its additional neutron, cobalt-60 undergoes radioactive decay releasing gamma radiation. Gamma radiation passes through different materials effortlessly and is used to sterilize medical equipment (e.g., gloves, syringes, cotton balls, etc.) by destroying harmful bacteria. In food preservation, gamma rays also destroy harmful bacteria, as well as fungi, molds, and insects, while leaving the food unchanged. This increases the shelf life of the food.

The U.S. Department of Energy has a comprehensive Web site that lists a large number of isotopes and their beneficial uses. This site is located at http://www.ne.doe.gov/isotopes/neIsotopes2f.html.

The downside to any application involving isotopes is how to safely dispose of the radioactive waste generated during processing. Any nuclear process involves the manufacture of nuclear waste whether they are low level (gloves, cotton balls, hospital gowns) or high level (spent nuclear fuel rods) waste products. Concern about the management of nuclear waste materials has caused much controversy and concern among government agencies, industrial, scientific and medical users and citizenry who have nuclear repository facilities in their areas. This becomes a real issue of risks versus benefits.

To learn more about radioactive wastes, go to http://www.nrc.gov/waste.html.

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Performance Benchmark P.12.A.8

Students know most elements have two or more isotopes, some of which have practical applications. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think that microwave ovens generate radioactivity.
This is a commonly held misconception by students and it may originate from our common use of the term “nuke” when we cook food in microwave ovens. Also, students incorrectly believe that all radiation must be ionizing radiation and therefore radioactive.
To learn more about student misconceptions associated with microwave ovens, go to http://www.who.int/peh-emf/publications/facts/info_microwaves/en/index.html.

2. Students incorrectly think that irradiated food is radioactive.
Several factors combine to heighten the public's anxiety about both the short-range and long-range effects of radioactivity. Perhaps the most important source of fear is the fact that radioactivity can't be detected by the average person. Furthermore, the effects of exposure to radioactivity might not appear for months or years or even decades. Irradiated food is one area in which confusion about beneficial uses of radioactivity has delayed use of technology to protect health.

To lean more about food irradiation, go to http://www.physics.isu.edu/radinf/food.htm.
http://flux.aps.org/meetings/YR00/OSS00/abs/S500.html#SC4.002

3. Many students do not understand that atoms cannot be changed from one element to another.

Students are often taught in physical science classes that matter cannot be created nor destroyed. While this is true in chemical reactions, nuclear reactions are quite another story. In cyclotrons and reactors, nuclear reactions can remove and add nuclear particles to atoms, and if protons are added or removed from the nucleus, then the elemental nature of the atom will change. Similarly, addition and removal of neutrons results in changing the isotope of the element.

For further information on misconceptions associated with nuclear reactions, go to http://www.ansto.gov.au/info/reports/radboyd.html.

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Performance Benchmark P.12.A.8

Students know most elements have two or more isotopes, some of which have practical applications. I/S

Sample Test Questions

1. Nuclear changes occur only
a. in the space outside the atom
b. in the space between electrons and the nucleus
c. within the electrons’ orbital
d. in atom’s nucleus

2. The nucleus of an atom consists of
a. protons and electrons
b. neutrons and electrons
c. protons and neutrons
d. protons and anti-protons

3. A chemical element is determined by
a. the number of electrons in an atom
b. the number of protons in an atom
c. the number of neutrons in an atom
d. the atom’s chemical bonds

4. In an isotope for a particular element,
a. the numbers of neutrons vary
b. the numbers of electrons vary
c. the numbers of protons vary
d. radioactivity is always emitted

5. Radioactivity occurs because
a. atoms have unstable nuclei
b. atoms need to gain protons
c. atoms need to lose electrons
d. atoms need more nucleons

6. Practical applications of radioactivity do NOT include
a. treating and diagnosing medical conditions
b. analyzing air and water
c. generating energy
d. cooking food quickly

Students know most elements have two or more isotopes, some of which have practical applications. I/S

Answers to Sample Test Questions
1. (d)
2. (c)
3. (b)
4. (a)
5. (a)
6. (d)

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Performance Benchmark P.12.A.8

Students know most elements have two or more isotopes, some of which have practical applications. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Isotopes of Pennies Lesson
The American Association for the Advancement of Science has a site called Science NetLinks. This site supports the Project 2061 Benchmarks for Science Literacy by providing a large variety of science lessons. In particular, this site has a lesson concerning isotopes that demonstrates how isotopes of a particular element have different masses. Pennies are used to simulate subatomic particles.

To access this lesson, go to http://www.sciencenetlinks.com/lessons.cfm?DocID=176.

2. Background on Atoms and Light Energy
NASA’s Imagine the Universe site includes lessons and background information on astrophysics topics. At the site, succinct background information on atomic structure, including isotopes, is provided. The information is peppered with activities for students.

This background information is found at http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-atoms.html

3. The Tooth Behind Stonehenge Lesson
The Public Broadcast System’s Secret of the Dead program has an associated lesson plan, called “The Tooth Behind Stonehenge,” which is intended to deepen student understanding of how isotopes are used to date fossilized remains. The lesson combines clips from the video, with an inquiry investigation.

To get the lesson outline, go to http://www.pbs.org/wnet/secrets/lessons/lp_stonehenge.html

4. The Table of Isotopes
The Lawrence Berkeley Laboratory has created an interactive periodic table of elements that provides detail isotope data. By clicking on a particular element in the interactive table, students get a table of that element’s isotopes, decay half-life, and radioactive decay mode. By further clicking on a table in the isotope, detailed information is provided on the quantities of radioactivity emitted by that isotope.

To access the table, go to http://ie.lbl.gov/education/isotopes.htm

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