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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

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

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

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

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

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

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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

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

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

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Content Benchmark P.12.A.8

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

Sample Test Questions

1st Item Specification: Know that isotopes of an element have different numbers of neutrons and the same number of protons.

Depth of Knowledge Level 1

  1. Nuclear changes in an atom occur only in the
    1. space around the atom.
    2. space between the electron and the nucleus.
    3. electron orbitals.
    4. nucleus.
  1. The nucleus of an atom consists of protons and neutrons. In an isotope the
    numbers of
    1. protons vary.
    2. neutrons vary.
    3. electrons vary.
    4. quarks vary.
  1. Which of the following subatomic particles is also known as the fingerprint
    of an atom?
    1. Electrons
    2. Protons
    3. Neutrons
    4. Nucleons
  1. Different isotopes of a particular element contain the same number of
    1. neutrons.
    2. electrons.
    3. protons.
    4. nucleons.
  1. The difference between carbon-12 and carbon-14 is the number of
    1. neutrons.
    2. nucleons.
    3. electrons.
    4. protons.
  1. Carbon-12 and carbon-14 have
    1. different numbers of protons.
    2. the same number of protons.
    3. different numbers of electrons.
    4. the same number of neutrons.

Depth of Knowledge Level 2

  1. A student is reading about uranium isotopes and doesn’t understand how they can be the same element. Your best answer is
    1. isotopes are not the same but are in the same family as the main element.
    2. uranium is unique and does not follow the definition of an isotope.
    3. isotopes all have the same number of protons which makes
      them the same element.
    4. the electrons do not change in isotopes even when forming
      positive or negative ions.
  1. Atoms of a certain isotope have 73 neutrons and a mass number of 123. Z
    What is its atomic number?
    1. 73
    2. 123
    3. 196
    4. 50
  1. If an isotope has 7 protons and a mass number of 15, how many neutrons does it have?
    1. 8
    2. 22
    3. 15
    4. 7
  1. The atomic number of an isotope is 17. If it has a mass number of 35, then how many neutrons does it have?
    1. 17
    2. 18
    3. 35
    4. 52
  1. Which of the following are isotopes of each other?
    (I) An atom with 17 protons and 18 neutrons?
    (II) An atom with atomic number 16 and atomic mass 32?
    (III) An atom with atomic number 16 and 18 neutrons?
    (IV) An atom with 17 protons and 20 neutrons?
    1. I and II
    2. III and IV
    3. II and III
    4. I and IV

2nd Item Specification: Describe practical applications of isotopes of various elements (e.g., hydrogen, deuterium and tritium), carbon-14, uranium-235 and colbalt-60).

Depth of Knowledge Level 1

  1. One practical application of radioactivity includes
    1. treating and diagnosing medical conditions.
    2. measuring air speed.
    3. health food additive.
    4. creating and modifying foods.
  1. Which of the following is a practical application of radioactivity?
    1. Generating energy
    2. Measuring rainfall
    3. Recycling plastic
    4. Recording traffic
  1. Natural radioactivity occurs because atoms
    1. have unstable nuclei.
    2. gain protons.
    3. lose electrons.
    4. gain nucleons.
  1. Cobalt-60 is an isotope
    1. used in paint pigments.
    2. used to irradiate food.
    3. used to manufacture television sets.
    4. found in microwave ovens.
  1. Heavy water is used as a coolant in nuclear power reactors. Which of the following is the isotope in heavy water?
    1. Cobalt-60
    2. Uranium-238
    3. Nitrogen-14
    4. Hydrogen-2
  1. Modern medicine employs a device known as a positron emission tomography (PET) that can detect the smallest of tumors. Which of the following isotopes is used in the PET scan?
    1. Rubidium-82
    2. Iodine-131
    3. Hydrogen-3
    4. Cobalt-60
  1. Which method would be MOST practical for shipping highly sensitive materials around the world?
    1. Use embedded radioactive isotopic tracers.
    2. Use armed guards to protect the materials.
    3. Transport using specialized aircraft.
    4. Use specialized cameras, heat sensing devices and guard dogs.
  1. Which of the following methods measures carbon-14 to accurately determine the age of ancient artifacts?
    1. X-ray crystallography
    2. Radiowave analysis
    3. Sonar analysis
    4. Accelerator mass spectrometry

Depth of Knowledge Level 2

  1. One characteristic of technectium-99 (an isotope of the element technectium) that makes it useful as a radiopharmaceutical is it
    1. lingers in the environment and can be reused.
    2. emits toxic gamma radiation which is harmful to others.
    3. has a short half-life minimizing the radiation dose for the patient.
    4. has to be contained for thousands of years in a nuclear repository.
  1. Only processed uranium is used in nuclear reactors in the United States because
    1. it is a cheaper way of accessing usable uranium.
    2. uranium contains several isotopes only one which is fissionable.
    3. uranium-238 is more abundant than the other isotopes.
    4. uranium can be recycled at specialized facilities.
  1. Now that super microbes are so prevalent, the most effective way to sterilize medical and surgical equipment is to
    1. wash with soap and water.
    2. use antibacterial products.
    3. irradiate with cobalt-60.
    4. use bleach in the rinse water.
  1. In medicine, why is it often important to be able to use radioisotopes
    which are short-lived?
    1. They are cheaper and easier to manage.
    2. The isotopes pass through the body quicker.
    3. They linger causing cancer.
    4. The isotopes are detected by radiotransmitters.
  1. What arguments support food irradiation by gamma rays?
    1. Isotopes can linger in the food causing cancer.
    2. Food products change their flavor.
    3. Gamma rays cannot be contained by human built containers.
    4. Harmful bacteria are killed preventing deaths by food poisoning.
  1. One societal implication of using radioisotopes in many everyday applications is
    1. short-lived isotopes are not useful.
    2. food supplies last longer.
    3. disposal of associated wastes.
    4. medical equipment decays faster.

3rd Item Specification: Know the meanings of subscripts and superscripts
in a nuclear symbol.

Depth of Knowledge Level 1

  1. The mass number of an isotope represents the
    1. total number of electrons and protons.
    2. number of neutrons.
    3. number of neutrons minus protons.
    4. total number of protons and neutrons.
  1. The mass number of an isotope is also known as the
    1. “Z” number.
    2. electron number.
    3. “A” number.
    4. neutron number.
  1. Examine the figure below.

The bottom number represents the number of

  1. nucleons.
  2. protons.
  3. neutrons.
  4. antiparticles.
  1. Examine the figure below.

The top number represents

  1. nucleons.
  2. protons.
  3. neutrons.
  4. antiparticles.

Depth of Knowledge Level 2

  1. Examine the figure below.

Which of the following is correct? Lithium has

  1. 4 protons, 7 neutrons and 3 electrons.
  2. 7 neutrons, 3 protons and 4 electrons.
  3. 10 nucleons, 3 electrons and 7 protons.
  4. 3 protons, 4 neutrons and 3 electrons.
  1. Examine the figure below.

Which of the following is correct? Lithium has

  1. 7 nucleons.
  2. 3 neutrons.
  3. 4 protons.
  4. 4 electrons.
  1. Examine the figure below.

Which of the following is correct? Lithium has

  1. 7 protons and electrons.
  2. 4 neutrons and 3 protons.
  3. 3 neutrons and 3 electrons.
  4. 7 protons and 4 nucleons.

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Content Benchmark P.12.A.8

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

Answers to Sample Test Questions

  1. D, DOK Level 1
  2. B, DOK Level 1
  3. B, DOK Level 1
  4. C, DOK Level 1
  5. A, DOK Level 1
  6. B, DOK Level 1
  7. C, DOK Level 2
  8. D, DOK Level 2
  9. A, DOK Level 2
  10. B, DOK Level 2
  11. D, DOK Level 2
  12. A, DOK Level 1
  13. A, DOK Level 1
  14. A, DOK Level 1
  15. B, DOK Level 1
  16. D, DOK Level 1
  17. A, DOK Level 1
  18. A, DOK Level 1
  19. D, DOK Level 1
  20. C, DOK Level 2
  21. B, DOK Level 2
  22. C, DOK Level 2
  23. B, DOK Level 2
  24. D, DOK Level 2
  25. C, DOK Level 2
  26. D, DOK Level 1
  27. C, DOK Level 1
  28. B, DOK Level 1
  29. A, DOK Level 1
  30. D, DOK Level 2
  31. A, DOK Level 2
  32. B, DOK Level 2

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Content 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

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

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

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

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