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11.6 End-of-Chapter Material

Chapter Summary

To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.

Some atoms have unstable nuclei that emit particles and high-energy electromagnetic radiation to form new elements that are more stable. This emission of particles and electromagnetic radiation is called radioactivity. There are three main types of spontaneous radioactive emission: alpha particles, which are equivalent to helium nuclei; beta particles, which are electrons; and gamma radiation, which is high-energy electromagnetic radiation. Another type of radioactive process is spontaneous fission, in which large nuclei spontaneously break apart into smaller nuclei and, often, neutrons. In all forms of radioactivity, new elements are formed from the radioactive reactants.

Radioactive isotopes decay at different rates. The rate of an isotope’s decay is expressed as a half-life, which is the amount of time required for half of the original material to decay. The length of its half-life is a characteristic of the particular isotope and can range from less than microseconds to billions of years.

Amounts of radioactivity are measured in several different ways. A becquerel is equal to one radioactive decay per second. A curie represents 3.7 × 1010 decays per second. Other units describe the amount of energy absorbed by body tissues. One rad is equivalent to 0.01 joule of energy absorbed per gram of tissue. Different tissues react differently to different types of radioactivity. The rem unit takes into account not only the energy absorbed by the tissues, but also includes a numerical multiplication factor to account for the type of radioactivity and the type of tissue. The average annual radiation exposure of a person is less than 360 millirem, over 80% of which is from natural sources. Radioactivity can be detected using photographic film or other devices such as Geiger counters.

Radioactive isotopes have many useful applications. They can be used as tracers to follow the journey of a substance through a system, like an underground waterway or a metabolic pathway. Radioactive isotopes can be used to date objects, since the amount of parent and daughter isotopes can sometimes be measured very accurately. Radioactive emission can be used to sterilize food for a longer edible lifetime. There are also a number of diagnostic and therapeutic medical applications for radioactive isotopes.

Radioactive processes occur with simultaneous changes in energy. This nuclear energy can be used to generate power for human use. Nuclear reactors use the energy released by fission of large isotopes to generate electricity. When carefully controlled, fission can produce a chain reaction that facilitates the continuous production of energy. If not carefully controlled, a very quick production of energy can result, as in an atomic bomb. Natural uranium does not contain enough of the proper isotope of uranium to work in a nuclear reactor, so it must first be enriched in uranium-235. Forcing small nuclei together to make larger nuclei, a process called fusion, also gives off energy; however, scientists have yet to achieve a controlled fusion process.

Additional Exercises

  1. Given that many elements are metals, suggest why it would be unsafe to have radioactive materials in contact with acids.

  2. Many alpha-emitting radioactive substances are relatively safe to handle, but inhaling radioactive dust can be very dangerous. Why?

  3. Uranium can be separated from its daughter isotope thorium by dissolving a sample in acid and adding sodium iodide, which precipitates thorium(III) iodide:

    Th3+(aq) + 3I(aq) → ThI3(s)

    If 0.567 g of Th3+ were dissolved in solution, how many milliliters of 0.500 M NaI(aq) would have to be added to precipitate all the thorium?

  4. Thorium oxide can be dissolved in an acidic solution:

    ThO2(s) + 4H+ → Th4+(aq) + 2H2O(ℓ)

    How many milliliters of 1.55 M HCl(aq) are needed to dissolve 10.65 g of ThO2?

  5. Radioactive strontium is dangerous because it can chemically replace calcium in the human body. The bones are particularly susceptible to radiation damage. Write the nuclear equation for the beta emission of strontium-90.

  6. Write the nuclear equation for the beta emission of iodine-131, the isotope used to diagnose and treat thyroid problems.

  7. A common uranium compound is uranyl nitrate hexahydrate [UO2(NO3)2_6H2O]. What is the formula mass of this compound?

  8. Plutonium forms three oxides: PuO, PuO2, and Pu2O3. What are the formula masses of these three compounds?

  9. A banana contains 600 mg of potassium, 0.0117% of which is radioactive potassium-40. If 1 g of potassium-40 has an activity of 2.626 × 105 Bq, what is the activity of a banana?

  10. Smoke detectors typically contain about 0.25 mg of americium-241 as part of the smoke detection mechanism. If the activity of 1 g of americium-241 is 1.26 × 1011 Bq, what is the activity of americium-241 in the smoke detector?

  11. Uranium hexafluoride (UF6) reacts with water to make uranyl fluoride (UO2F2) and hydrogen fluoride (HF). Balance the following chemical equation:

    UF6 + H2O → UO2F2 + HF
  12. The cyclopentadienyl anion (C5H5) is an organic ion that can make ionic compounds with positive ions of radioactive elements, such as Np3+. Balance the following chemical equation:

    NpCl3 + Be(C5H5)2 → Np(C5H5)3 + BeCl2

Answers

  1. Acids can dissolve metals, making aqueous solutions.

  2. 14.7 mL

  3. S3890re10+Y3990

  4. 502 g/mol

  5. about 18 Bq

  6. UF6 + 2H2O → UO2F2 + 4HF