Atomic structure

• Positive nucleus


  • Protons & neutrons
  • 1/10,000 of an atom
• Protons


  Charge=+1, Mass=1
• Neutrons


  Charge=0, Mass=1
• Electrons


  Charge=-1, Mass=1/2000
• Ions


  Atoms that have gained or lost electrons & now have a charge
• Isotopes


  Atoms of the same element with the same number of protons & electrons & a different number of neutrons (different mass)
• Electrons arranged at different distances from the nucleus


  1. By energy levels

  2. Energy level changes when the atom emits or absorbs electromagnetic radiation (absorbs=further, emits=closer)
• Early atom models


  • Atoms thought to be tiny spheres that couldn't be divided into smaller particles
• Plum pudding model


  • Sea of positive charge with random electrons embedded inside
• Geiger & Marsden experiment

  • Gold leaf bombarded with alpha particles, the positive particles should have been deflected back towards the source but only a tiny number did with most passing through
• Nuclear model


  • Most positive charge was concentrated in a small area named the nucleus with mostly empty space around containing electrons
• Bohr atom


  • Electrons must orbit at specific distances (in shells) or they would spiral inwards
• Atomic model


  • Evidence provided to show neutrons & protons could be found in the nucleus
• Nuclear decay & radiation


  Atomic nuclei can be unstable so give out radiation to become more stable
• Type of radiation


  Dependent on why the nucleus is unstable
• Activity


  Measured in sieverts or becquerels (Bq)
• Count rate


  Number of decays a second recorded by a detector (Geiger-Muller tube)
• 1Bq


  1 decay per second = 1 count per second
• Types of nuclear radiation


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


  • Helium nucleus (2 protons & 2 neutrons), +2 charge, ejected from the nucleus, most ionising, least penetrable, absorbed by a few cm of air or paper, highly likely to be absorbed & cause damage if passing through living cells, most magnetic
• Beta (β)


  • High speed electron, ejected from nucleus as a neutron turns into a proton, reasonably ionising, -1 charge, travels ≈ 1m in air, passes through paper, absorbed by a few mm of aluminium, likely to cause damage if absorbed by cells, can penetrate body to organs, less magnetic
• Gamma (γ)


  • Electromagnetic (high frequency) wave, emitted by nucleus, least ionising, most penetrating, many cm of lead or many m of concrete needed to absorb, likely to pass through cells without being absorbed or ionising, not magnetic
• Neutron (N)


  4th type of radiation emitted during decay
• Contamination


  Unwanted presence of materials containing radioactive atoms on other materials, gives out radiation until decontaminated
• Contamination hazards


  Depend on the type of radiation, food contaminated with alpha source is more dangerous than gamma because of ionisation
• Irradiation


  Exposure to radiation deliberate or accidental, doesn't cause the object to be radioactive
• To prevent unwanted irradiation


  1. Use sources with the lowest activity possible for the least amount of time

  2. Wear appropriate protective clothing (lead apron)

  3. Not handle sources with bare hands
• Half-life


  Average time taken for half of the nucleus to decay (count rate ½), doesn't change in a particular isotope, in different isotopes half-life can vary from seconds to millions of years
• Short half-lives


  • Unstable, emit radiation quickly, exposure hazardous, not radioactive for long
• Long half-lives


  • More stable, radioactive for longer, emit slowly, exposure less hazardous
• Uses of radiation


  • Smoke detectors (α)
  • Testing thickness of metal sheets (β)
  • Medical tracers
  • Treatment of tumours (γ)
  • Sterilisation & disinfection (γ)
• When choosing which isotope to use


  1. Consider half life (long enough for job, short enough to prevent damage)

  2. Activity & type of radiation

  3. Studies into the effect of radiation on the body allow scientists to weigh up benefits & risks
• Sources of background radiation


  • Natural (87%)
  • Human (13%)
• Human sources


  • Medical (x-rays etc.)
  • Nuclear industry
• Nuclear fission


  Splitting of a large unstable nucleus (uranium or plutonium)
• Nuclear fission process


  1. Nucleus must absorb neutron before it starts

  2. The nucleus splits into 2 smaller nuclei (≈½) & emits 2 or 3 neutrons, gamma rays & energy

  3. The neutrons then start a chain reaction when absorbed by nuclei

  4. Chain reaction controlled to give constant, steady energy in a reactor
• Nuclear fusion


  Joining of 2 light nuclei to form a heavier nucleus, some mass converted into energy & emitted as radiation
• Nuclear fusion


  Requires high temperature to overcome electrostatic repulsion & bring them together
• Sum of mass numbers must be the same on both sides of nuclear equations
• Sum of atomic numbers must be the same on both sides of nuclear equations