P7 - Radioactivity

• Radioactive substance


  A substance that emits radiation
• Types of radiation emitted from a radioactive substance

  • Alpha (α)
  • Beta (β)
  • Gamma (γ)

  
• Radioactive decay


  When a radioactive source emits radiation

  
• There are different types of radiation emitted by radioactive sources

  
• Becquerel's discovery


  • Mysterious image on photographic film under a key and uranium salts
  • Uranium salts emitted radiation that passed through paper but not metal

  
• Radioactivity


  The strange new property of uranium discovered by Marie Curie
• Marie Curie and Pierre Curie did more research into radioactivity and discovered new radioactive elements, including polonium
• Geiger counter


  A device used to detect radioactivity, made up of a Geiger-Müller tube and an electronic counter
• Safety: Avoid touching and inhaling radioactive material
• Alpha (α) radiation


  • Stopped by paper
  • Made up of positively charged particles

  
• Beta (β) radiation


  • Went through paper

  
• Gamma (γ) radiation


  • More penetrating than beta radiation
• Rutherford's investigations


  1. Directed a narrow beam of alpha particles at a thin metal foil

  2. Observed that some alpha particles rebounded from the foil

  
• Nucleus


  The positively charged centre of an atom, containing most of the atom's mass
• Protons and neutrons


  The particles that make up the nucleus
• Unstable nucleus


  A nucleus that becomes stable or less unstable by emitting alpha, beta, or gamma radiation

  
• Radioactive decay is a random event that happens without anything being done to the nucleus

  
• Rutherford's nuclear model of the atom

  • Most alpha particles passed straight through the metal foil
  • Number of alpha particles deflected decreased as angle of deflection increased
  • About 1 in 10,000 alpha particles were deflected by more than 90°

  
• Plum pudding model of the atom

  Atoms contain tiny negatively charged particles (electrons) evenly spread in a positively charged matter
• Bohr's model of the atom

  • Electrons orbit the nucleus at specific distances and energy levels
  • Electrons can move to different orbits by absorbing or emitting electromagnetic radiation

  
• The hydrogen nucleus has the least amount of charge
• The charge of any nucleus is shared equally between a whole number of smaller particles, each with the same amount of positive charge
• Proton


  The positively charged particle in the nucleus, named after the hydrogen nucleus
• Neutron


  The uncharged particle with about the same mass as a proton, found in the nucleus of atoms except hydrogen
• Isotope


  Atoms of the same element with different numbers of neutrons
• Atomic number (Z)


  The number of protons in a nucleus
• Mass number (A)


  The number of protons plus neutrons in a nucleus
• Alpha (α) particle emission

  1. Reduces the atomic number by 2 and the mass number by 4

  2. Reduces the mass and charge of the nucleus

  
• Beta (β) particle emission


  1. Increases the atomic number by 1 and leaves the mass number unchanged

  2. Increases the charge of the nucleus and leaves the mass unchanged

  
• The numbers along the top show that the total number of protons and neutrons after the change (= 224 + 4) is equal to the total number of neutrons and protons before the change (= 228)
• The numbers along the bottom show that the total number of protons after the change (= 88 + 2) is equal to the total number of protons before the change (= 90)
• B particle


  An electron created and emitted by a nucleus that has too many neutrons compared with its protons. A neutron in the nucleus changes into a proton and a B particle (i.e. an electron), which is instantly emitted

  
• Relative mass of a B particle

  Effectively zero
• When an unstable nucleus emits a B particle, the atomic number of the nucleus goes up by 1, and its mass number is unchanged (because a neutron changes into a proton)

  
• When an unstable nucleus emits a B particle, the charge of the nucleus is increased, and the mass of the nucleus is unchanged

  
• The numbers along the top show that the total number of protons and neutrons after the change (= 40 + 0) is equal to the total number of neutrons and protons before the change (= 40)
• The numbers along the bottom show that the total charge (in relative units) after the change (= 20 - 1) is equal to the total charge before the change (= 19)
• Gamma ray


  Electromagnetic radiation from the nucleus of an atom. It is uncharged and has no mass. So its emission does not change the number of protons or neutrons in a nucleus. So the mass and the charge of the nucleus are both unchanged

  
• Neutron emission


  Neutrons are emitted by some radioactive substances as a result of particles colliding with unstable nuclei in the substance. Such a collision causes the unstable nuclei to become even more unstable and emit a neutron. Because the emitted neutrons are uncharged, they can pass through substances more easily than an alpha particle or a beta particle can

  
• A B particle is created in the nucleus and instantly emitted