P4: Atomic Structure

• Atomos


  Identical lumps that all matter was made up of, according to early Greek philosophers
• Democritus proposed that all matter was made up of tiny "atomos"


  5th Century BC
• Plum Pudding Model


  Atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
• Alpha Scattering Experiment


  1. Firing a beam of alpha particles at thin gold foil

  2. Most particles passed straight through

  3. Some were deflected more than expected

  4. A few were deflected back the way they had come
• Findings from Alpha Scattering Experiment

  • Most of the mass of the atom must be concentrated at the centre in a tiny nucleus
  • The nucleus must also have a positive charge, since it repelled the positive alpha particles
  • Most of an atom is just empty space
• Nuclear Model of the Atom

  A positively charged nucleus surrounded by a cloud of negative electrons
• Bohr's Theoretical Calculations


  1. Electrons orbit the nucleus at certain distances called energy levels

  2. Agreed with experimental data
• Improved Nuclear Model


  • Nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus
  • Existence of neutrons proved, which explained the imbalance between atomic and mass numbers
• Current Model of the Atom

  • Tiny nucleus containing protons and neutrons, surrounded by a cloud of fast-moving electrons
  • Nucleus makes up most of the mass of the atom
  • Electrons in energy levels can move within or leave the atom
• The current model of the atom may change in the future as new experiments are conducted
• Isotopes


  Atoms with the same number of protons (atomic number) but different numbers of neutrons (mass number)
• Radioactive Decay


  Unstable isotopes decay into other elements and give out radiation to become more stable
• Types of Ionising Radiation

  • Alpha
  • Beta
  • Gamma
• Alpha Particles


  Helium nuclei emitted from the nucleus, strongly ionising but not very penetrating
• Beta Particles


  Fast-moving electrons emitted from the nucleus, moderately ionising and moderately penetrating
• Gamma Rays


  Electromagnetic radiation emitted from the nucleus, weakly ionising but highly penetrating
• When an atom emits an alpha particle


  Its atomic number decreases by 2 and its mass number decreases by 4
• When an atom undergoes beta decay

  A neutron turns into a proton, increasing the atomic number by 1 but not changing the mass number
• Gamma rays do not change the charge or mass of the nucleus
• Uses of Radioactive Radiation

  • Alpha radiation in smoke detectors
  • Beta radiation to test metal sheet thickness
  • Gamma radiation for various uses
• Nuclear Equations


  Show radioactive decay, with the total mass and atomic numbers equal on both sides
• Alpha decay


  A type of radioactive decay where an atomic nucleus emits an alpha particle
• Gamma rays are sometimes also released when a nucleus decays by alpha or beta decay
• Beta decay


  1. Neutron in the nucleus turns into a proton and releases a fast-moving electron (the beta particle)

  2. The number of protons in the nucleus has increased by 1

  3. The mass of the nucleus doesn't change as protons and neutrons have the same mass

  4. A beta particle is written as e in nuclear equations
• In both alpha and beta emissions a new element will be formed, as the number of protons (atomic number) changes
• Gamma rays don't change the charge or mass of the nucleus
• Gamma rays


  A way of getting rid of excess energy from a nucleus
• Radioactivity


  A totally random process where radioactive substances give out radiation from the nuclei of their atoms
• Activity


  The rate at which a radioactive source decays, measured in becquerels (Bq) where 1 Bq = 1 decay per second
• The radioactivity of a source decreases over time as radioactive nuclei decay
• Half-life


  The time taken for the number of radioactive nuclei in an isotope to halve
• The half-life is the time taken for the activity, and so count-rate, to halve
• A short half-life means the activity falls quickly, while a long half-life means the activity falls more slowly
• A graph of activity against time will always be shaped like an exponential decay curve
• The half-life can be found from the graph by finding the time interval corresponding to a halving of the activity
• Background radiation


  Low-level radiation that's around us all the time, from natural sources and human activity
• Background radiation comes from naturally occurring unstable isotopes, radiation from space, and radiation due to human activity
• Radiation dose


  The risk of harm to body tissues due to exposure to radiation, measured in sieverts (Sv)
• Exposure to radiation is called irradiation, which does not make an object radioactive
• Contamination


  Radioactive particles getting onto or into an object, which can then decay and cause harm