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Radioactivity (2nd half of Ch 39) 39.5 Radioactive Half-Life Radioactive isotopes decay at different rates. The radioactive decay rate is measured in terms of a half ... – PowerPoint PPT presentation

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Title: Radioactivity

  • (2nd half of Ch 39)

39.5 Radioactive Half-Life
  • Radioactive isotopes decay at different rates.
  • The radioactive decay rate is measured in terms
    of a half-life.
  • The half-life of a substance is the time needed
    for half of the radioactive atoms to decay.
  • Some half-lives are less than a millionth of a
    second. But some, like Uranium-238, has a
    half-life of 4.5 billion years.

39.5 Half-Life cont.
  • If half-lives are so long, how do scientists
    measure them?
  • They can measure the rate at which a substance
    decays, using radiation detectors.
  • The half-life of an isotope is related to its
    rate of disintegration (a half-life can be
    computed from the rate of disintegration.)
  • Stop answer question 1 on pg 616!

39.6 Transmutation
  • When a nucleus emits an alpha or beta particle, a
    different element is formed.
  • The changing of one element to another is called
  • For example Uranium has 92 protons. When an
    alpha particle is ejected during decay, the
    nucleus is reduced by two protons and two
    neutrons. The new element is Thorium (atomic
    number 90).

Transmutation cont.
  • The Thorium is also radioactive, and decays by
    Beta particles.
  • When a beta particle is ejected, one of the
    neutrons spontaneously changes into a proton.
  • The new element is now Protactinium (atomic
    number 91).
  • Eventually, Uranium decays to harmless and
    non-radioactive lead (atomic number 82). See
    Figure 39.15 pg 619.
  • STOP Answer question 2 on pg 618.

39.7 Artificial Transmutation
  • Ernest Rutherford in 1919 was the first to
    succeed in artificially transmuting an element.
  • He bombarded nitrogen nuclei with alpha particles
    and found oxygen and hydrogen atoms that were not
    there before.
  • Artificial transmutation can be used to convert
    harmful radioactive waste into nonradioactive

39.8 Carbon Dating
  • Taking carbon out to Applebees. (hahaha) ?
  • When cosmic rays from outer space hit our upper
    atmosphere, transmutation takes place and
    protons, neutrons and electrons are created.
  • The protons combine with electrons to make
    hydrogen, but the neutrons find their way down to
    Earths lower atmosphere and react with Nitrogen
    to form Carbon-14 and Hydrogen.

39.8 Carbon Dating cont.
  • Plants take in some of this radioactive
  • Animals eat plants (or eat other animals that eat
    plants), so animals get Carbon-14 in them.
  • Carbon-14 is a beta emitter, and decays back into
  • When an animal or plant dies, the amount of
    Carbon-14 it has stops being replenished.
  • The Carbon-14 in the dead plant or animal decays
    at a known rate of 5730 years for a half-life.
  • The ratio of regular Carbon-12 to Carbon-14 can
    then be used to determine how long ago the
    organism lived.
  • STOP Answer question 2 on pg 622.

39.9 Uranium Dating
  • The dating of older, non-living things is
    accomplished with uranium instead of Carbon-14.

39.10 Radioactive Tracers
  • Small amounts of radioactive elements can be
    added to things to determine how well that thing
    is working.
  • Examples it can be added to fertilizer to see
    how much is being taken up into the plants,
    medicine to see how much is being absorbed,
    sewage lines to see where the leak in the
    underground pipe is, etc.
  • See Figure 39.19 pg 624.

39.11 Radiation and You
  • Radiation has been around since before humans.
  • Radiation is what warms the planet and keeps the
    core molten.
  • Even the helium in a balloon is the product of

39.11 Radiation and You cont.
  • Much of the radiation we are exposed to is cosmic
    radiation from space.
  • The atmosphere stops a lot of this radiation.
  • At higher elevations, radiation is more intense.
    For example, in Denver, you get twice the
    radiation than you do at sea level.
  • A couple of flights between NY and San Francisco
    expose you to as much radiation as a chest X-ray.

39.11 Radiation and You cont.
  • Radiation damages cells.
  • Cells can repair most kinds of molecular damage
    if the radiation the cells are exposed to is not
    too intense.
  • But if you are exposed to intense radiation, or
    moderate radiation over a long period of time,
    cancer and a shorter life expectancy can result.

  • Worksheet!