Alpha-Scattering and the Nuclear Model

• Atom


  Tiny sphere that cannot be divided
• In 1897, scientists discovered that atoms contain tiny negative particles called electrons
• Plum pudding model


  Atom is a ball of positive charge with negative electrons embedded in it
• Alpha scattering experiment


  1. Fired alpha particles at gold foil

  2. Most alpha particles passed straight through

  3. Some alpha particles were deflected

  4. Some alpha particles bounced straight back
• Results of alpha scattering experiment

  • Atoms are mainly empty space
  • Center of atom has positive charge that repelled alpha particles
  • Mass of atom is concentrated in the center
• Nuclear model


  Most of atom is empty space, with a tiny positive nucleus containing almost all the mass, and negative electrons orbiting the nucleus
• Electron energy levels/shells


  Electrons orbit the nucleus at specific distances
• Positive charge in nucleus is due to positive particles called protons
• Nucleus also contains neutral particles called neutrons
• Isotopes


  Different forms of an element, with the same number of protons but different numbers of neutrons
• Only one or two of an element's isotopes are stable, while the rest are unstable and undergo radioactive decay</b>
• Radioactive


  Consisting of unstable isotopes that can decay
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Alpha particles


  • Made up of two protons and two neutrons, the same as a helium nucleus
  • Have an overall charge of two plus
  • Easily stopped by collisions, can only travel a few centimetres in air and are absorbed by a single sheet of paper
  • Strongly ionizing, can easily knock electrons off atoms they collide with
• Beta particles


  • Just electrons
  • Emitted when a neutron in the nucleus decays into a proton and an electron
  • Have a charge of -1 and virtually no mass
  • Moderately ionizing and penetrate moderately far, stopped by several metres of air or 5mm of aluminium
• Gamma rays


  • Waves of electromagnetic radiation, not particles
  • Often emitted after alpha or beta radiation to get rid of extra energy
  • Have no mass or charge, so tend to pass straight through materials
  • Only weakly ionizing, can penetrate far into materials and require thick lead or concrete to stop
• Neutron emission


  • Occurs when a nucleus contains too many neutrons, making it unstable
• Alpha particles are stopped by a single sheet of paper, beta particles by a thin sheet of aluminium, and gamma rays require a thick piece of lead to stop them
• Nuclear equations for alpha, beta, and gamma radiation

  1. Alpha particles consist of two protons and two neutrons

  2. Unstable nucleus undergoes alpha decay and emits alpha particle

  3. Subtract 4 from mass number and 2 from atomic number

  4. Resulting element is different, check periodic table

  5. Add emitted helium
• Beta decay

  1. Neutron turns into proton and emits fast moving electron (beta particle)

  2. Atomic number increases by 1, mass number stays same

  3. Represent beta particle with beta symbol and charge of -1
• Gamma radiation

  1. Gamma radiation is pure energy, no mass or charge

  2. Does not change anything in the nucleus
• Neutron emission

  Subtract 1 from mass number to account for lost neutron