Half-Life

• Key Point

  Half-life is a measure of how long it takes for half of the radioactive atoms to decay.
• Half-Life

  . The random nature of radioactive decay makes it impossible to predict which nucleus will decay next.

  . However, with a large enough number of nuclei, it is possible to predict how many will decay in a certain time period.

  . The half-life of a radioactive isotope is:

  - the average time it takes for half of the nuclei to decay

  - the time it takes for the count rate, or activity, of a sample containing the isotope to fall to 50% of its original value.

  . If the half-life is known, then the activity of a source after a certain amount of time can be calculated.
• What are nuclear equations used to represent?


  Radioactive decay
• What is the symbol for an alpha particle?


  ⁴₂He
• What is the symbol for a beta particle?


  e (with 0 on top and -1 on bottom)
• What happens to the mass number of an element when an alpha particle is emitted?


  It is reduced by 4
• What happens to the atomic number of an element when an alpha particle is emitted?


  It is reduced by 2
• Why is the mass number reduced by 4 during alpha decay?


  Because 2 protons and 2 neutrons are emitted from the nucleus
• What happens to the atomic number during beta decay?


  It is increased by 1
• Why does the atomic number increase by 1 during beta decay?


  Because a neutron turns into a proton and an electron
• What is emitted as the beta particle during beta decay?


  An electron
• What effect does the emission of a gamma ray have on the nucleus?


  It does not cause a change in mass or charge
• What are the requirements for writing balanced decay equations for alpha and beta decay?


  • Mass numbers on the right-hand side must equal those on the left
  • Atomic numbers on the right must equal those on the left
• What happens to the mass number during beta decay?


  It does not change