SP6

• Plum pudding model


  -the atom being the pudding

  -the plum being the electrons scattered around
• Rutherford's experiment- observations

  1. most alpha particles went straight through the foil
  2. Some alpha particles got deflected at small angles
  3. Some alpha particles bounced back in the same direction
• Isotopes are atoms with different numbers of neutrons but same number of protons.
• Rutherford's conclusions

  1. The atom is mostly empty space
  2. The nucleus is positively charged
  3. Most of the atoms mass is concentrated at the centre
• Atomic number


  the number of protons

  *top one
• mass number


  number of protons and electrons

  *bottom one
• protons


  -charge is 1+

  -relative mass 1
• neutrons


  -charge 1

  -mass 1
• electron 


  -charge -1

  -mass= 1/1835
• Nucleon


  protons and neutrons
• Bohr's model

  • electron radii are fixed
  • electrons need to gain or lose energy to move between shells
  • The colour emitted depends on the amount of energy lost when electrons move shells
  • The colours shown are on the emission spectrum
• Ionisation


  Radiation that causes electrons to escape is called ionising radiation

  Atoms can gain so much energy that they can escape from the atom
• Background radiation


  Naturally occurring environmental radiation
• Natural sources of background radiation

  1. Cosmic rays= reaches earth from space
  2. Rocks and soil= some rocks give off radon gas
  3. Living things=passed up food chain by plants absorbing radation
• Artificial sources of background sources

  1. Radioactive waste
  2. Medical X-rays
• Geiger-Muller tube


  The count rate is the number of clicks per second
• Alpha radiation

  • Helium nucleus
  • Low at penetrating
  • High at ionising
• Beta radiation

  1. High energy electron
  2. medium at both penetrating and ionising
• Gamma radiation

  1. High frequency wave
  2. Highly penetrating
  3. Low ionising
• Beta minus decay


  Decays into a new element by changing a neutron into a proton and electron
• Beta plus decay


  A proton into a neutron and positron
• Neutron decay


  Nucleus changes to form isotopes of the same element
• The activity


  of a radioactive substance is the number of the nuclear decays per seconds

  *Measured in Bq
• Half-life


  the amount of time it takes for half of the unstable nuclei to decay
• Gamma radiation uses

  1. Sterilising food= food is irradiated with gamma rays to make it last longer
  2. Sterilising medical equipment
• Alpha radiation uses


  Smoke alarm
• How smoke alarms work?

  1. Radioactive sources gives off alpha particles
  2. Particles ionise air molecules, then move across the gap creating a currant
  3. Smoke will slow down ions breaking currant
  4. The detector senses and makes siren sounds
• Beta radiation uses


  Paper industry

  -emits beta particles either side

  -paper too thick= not enough beta particles detector on other side, most pressure on rollers

  -too thin rollers reduce force
• Tracers


  Radioactive isotopes can be used as tracers

  E.g. gamma source added to water pipes to detect leaks with a geiger muller tube
• Contamination


  Radioactive substance is either in the body or on your skin
• Dosimeter


  Dosimeter badges used to monitor exposure
• Irradiated


  Exposed to radiation
• Nuclear fission

  1. neutron absorbed by an atom e.g. U235
  2. U235 becomes an unstable isotope U236
  3. Splits into two daughter nuclei and three neutrons
  4. Process repeats
• Chain reaction


  Nuclear fission releases large amounts of energy

  -we use this to drive turbines to generate energy
• Nuclear fission- control rods


  Absorb the neutrons to control the chain reaction
• Nuclear fission- moderator


  Slows down neutrons so that the neutrons hit the Uranium fuel and fission continues
• How nuclear power stations produce energy?

  1. Fission chain reactions are used to generate heat
  2. The heat boils the water
  3. Steams turns the turbine
  4. Turbine turns a generator which generates electricity