Ch 9

• Radioactivity

  Process by which atoms emit energetic particles or rays
• Radiation

  The particles or rays emitted, come from the nucleus
• Nuclear symbols

  What we use to describe the nucleus
• Atomic symbol

  Symbol for the element
• Atomic number

  Number of protons in the atom
• Mass number

  Number of protons and neutrons in the atom
• Isotope

  Atoms of the same element with different numbers of neutrons
• Nuclide

  In nuclear chemistry, an isotope
• Isotopes of boron

  • Boron-10
  • Boron-11
• Each nucleus of carbon isotopes contains the same number of protons, only the number of neutrons is different
• Stable isotopes


  Isotopes that do not undergo radioactivity
• Unstable isotopes

  Isotopes that produce radioactivity
• Alpha particles

  2 protons, 2 neutrons, slow moving, stopped by small barriers
• Beta particles

  Fast-moving electrons emitted from the nucleus as a neutron is converted to a proton
• Positrons

  Particles with the same mass as an electron but opposite charge (+)
• Gamma rays

  Pure energy (electromagnetic radiation), highly energetic, most penetrating form of radiation
• Ionizing radiation

  • Produces a trail of ions throughout the material it penetrates
  • The penetrating power determines the ionizing damage caused (alpha < beta < gamma)
• Nuclear equation

  Used to represent nuclear change
• Writing a balanced nuclear equation

  1. The sum of the mass numbers on each side must be identical

  2. The sum of the atomic numbers on each side must be identical
• Alpha decay

  Mass number decreases by 4, atomic number decreases by 2
• Beta decay

  One neutron in the parent nuclide is converted to a proton, and a beta particle is released
• Positron emission

  The product nuclide has the same mass number as the parent but the atomic number has decreased by one
• Gamma production

  1. Gamma radiation occurs to increase the stability of an isotope

  2. The atomic mass and number do not change

  3. Gamma rays are usually emitted along with alpha or beta particles
• To predict the product of nuclear decay, the mass number and atomic number are conserved
• Binding energy

  • The energy that holds the protons, neutrons, and other particles together in the nucleus
  • When a nuclide decays, some energy is released because the products are more stable
• Factors for stable radioisotopes

  • Nuclear stability correlates with the ratio of neutrons to protons
  • Isotopes with even numbers of protons or neutrons are generally more stable
  • Isotopes with more protons than neutrons are unstable
• Half-life

  • The time required for one-half of a given quantity of a substance to undergo change
  • Each radioactive isotope has its own half-life, ranging from a fraction of a second to a billion years
  • The shorter the half-life, the more unstable the isotope
• The decay curve for the medically useful radioisotope Tc-99m is provided
• Predicting the extent of radioactive decay

  1. Calculate the number of half-lives elapsed

  2. Calculate the amount remaining after the elapsed half-lives
• Radiocarbon dating

  The estimation of the age of objects through measurement of the ratio of carbon-14 to carbon-12
• Basis for radiocarbon dating

  1. Carbon-14 is constantly being produced by cosmic radiation

  2. Living systems take in carbon, maintaining a constant ratio of carbon-14 to carbon-12

  3. After death, the amount of carbon-14 decreases according to its half-life
• Radiocarbon dating is limited to objects less than 50,000 years old (9 half-lives of carbon-14)
• Isotopes with a “magic number” of 2, 8, 20, 50,82, or 126 protons or neutrons are stable
• Nuclei with large number of protons (84 ormore) tend to be unstable
• For light atoms a ratio of 1 is typically stable
• All isotopes (except 1 H) with more protons thanneutrons are unstable; the opposite is not true.
• The half-life of carbon-14 is 5730 years
• The carbon-12 does not decay; it is the stableisotope