7'1 Atomic Theory and Radioactive Decay

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7.1 Atomic Theory and Radioactive Decay

• Natural background radiation exists all around
  us.
• Radioactivity is the release of high energy
  particles or waves that we call radiation
• When atoms lose high energy particles and waves,
  new atoms can be formed.
• Radiation can be good or bad
• X-rays, radiation therapy and electricity
  generation are beneficial.
• High energy particles and waves can do damage to
  DNA in our cells.

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The Electromagnetic Spectrum

• Early discoveries of radiation relied on
  photographic equipment
• Marie Curie and her husband Pierre
• named the energy radioactivity

Radium salts, after being placed on a
photographic plate, leave behind the dark traces
of radiation.
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Isotopes

• are atoms of the same element, with a different
  number of neutrons.
• changing the of neutrons changes the mass
  number
• Remember mass protons neutrons
• isotopes still have the same number of protons
  and the same element symbol

Atomic Mass (the decimal s)

• Atomic mass average of the mass numbers for all
  isotopes of an element.

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Representing Isotopes

• Isotopes are written two ways
• with the mass number at the end Ex. Potassium
  40
• With its chemical symbol Ex.

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Radioactive Decay

• Can result in new atoms forming.
• Radioactivity results from having an unstable
  nucleus.
• Radioactive decay when nuclei break apart
  release energy from the nucleus.
• Radioactive decay continues until a stable
  element forms.
• An element may have isotopes that are radioactive
  called radioisotopes
• Ex. carbon-12, carbon-13 and carbon-14
• (only C-14 is radioactive)

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Uranium goes through many decay steps before it
becomes stable
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• Rutherford identified three types of radiation
  using an electric field.
• Positive alpha particles were attracted to the
  negative plate.
• Negative beta particles were attracted to the
  positive plate.
• Neutral gamma particles did not move towards any
  plate.

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Alpha Radiation

• is a stream of alpha particles, (shown as ?)
• positively charged
• weighs the most
• are the same as a helium nucleus
• Alpha particles are represented by the symbols
• 2 protons and 2 neutrons make a mass number of 4
• it has a charge of 2 because of the protons

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• Alpha particles are big and slow. A sheet of
  paper will stop an alpha particle.
• Example the alpha decay of Radium - 226

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Beta Radiation

• A Beta particle, ?, is a high energy electron.
• negatively charged, and weigh less than alpha
  particles.
• Beta particles are represented by the symbols
• electrons are very tiny, so beta particles are
  assigned a mass of 0.
• one electron gives a beta particle has a charge
  of 1.

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• Beta decay occurs when a neutron changes into a
  proton an electron.
• The proton stays in the nucleus, and the electron
  is released.
• Example The beta decay of iodine - 131
• It takes a thin sheet of aluminum foil to stop a
  beta particle.

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Gamma Radiation

• Gamma radiation, ?, is a ray of high energy,
  short-wavelength radiation.
• has no charge and no mass.
• is the highest energy form of electromagnetic
  radiation.
• It takes thick blocks of lead or concrete to stop
  gamma rays.
• Gamma decay results from energy being released
  from a high-energy nucleus.

Shows unstable nucleus for gamma decay
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• Often, other kinds of radioactive decay will also
  release gamma radiation.
• Uranium-238 decays into an alpha particle and
  also releases gamma rays.

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Nuclear Equations

• are written like chemical equations, but
  represent changes in the nucleus of atoms.
• Chemical equations represent changes in the
  position of atoms, not changes to the atoms
  themselves.
• Remember
• The sum of the mass numbers on each side of the
  equation should equal.
• The sum of the charges on each side of the
  equation should equal.

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Summary Tables
Take the Section 7.1 Quiz