Physics P4

  • Created by: abbiedye
  • Created on: 29-07-18 11:04

What is an isotope?

  • Atoms of an element with the same number of protons (same atomic number and charge of the nucleus) but a different number of neutrons (different mass number)
  • All elements have different isotopes but only one or two are stable
  • The others decay and give out radiation
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Current model of the atom

  • Nucleus = tiny - most mass though, protons (positive, +1) and neutrons (neutral, 0) positive charge, 10,000 times smaller than the atom
  • Electrons = relative charge (-1), give the atom the overall size, can move within or leave the atom, gain energy from EM radiation - higher energy levels
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How was the model of the atom developed?

  • John Dalton - agreed with Democritus - atoms were tiny spheres - couldn't be broken
  • JJ Thompson - plum pudding - positive 'dough' with a lot of negative electrons
  • Rutherford - alpha particle scattering - deflected - concentrated mass in centre but mostly empty space
  • Niels Bohr - a cloud of negative electrons - orbit in shells - nucleus = protons
  • James Chadwick - neutron - nucleus
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What is alpha radiation?

  • When an alpha particle is emitted from the nucleus. An alpha particle is two neutrons and two protons (helium nucleus)
  • They don't penetrate very far and are stopped quickly. They can only travel a few cms in air and are absorbed by a sheet of paper
  • Size - strongly ionising
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What is beta radiation?

  • Fast moving electron released by the nucleus - no mass and a charge of -1
  • Moderately ionising - penetrate moderately far into materials before colliding and have a range in air of a few metres - absorbed by a sheet of aluminium (5mm)
  • For every beta particle emitted, a neutron in the nucleus has turned into a proton
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What is gamma radiation?

  • Gamma rays are waves of electromagnetic radiation released by the nucleus
  • Penetrate far into materials without being stooped and will travel a long distance through the air - weakly ionising, tend to pass through rather than collide with atoms - eventually they hit something and do damage
  • They can be absorbed by thick sheets of lead or metres of concrete
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What are nuclear equations?

  • A way of showing radioactive decay by using element symbols
  • Written in the form - atom before decay -> atom after decay + radiation emitted
  • Rule = total mass and atomic numbers must be equal on both sides
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What is alpha decay?

  • Alpha particle - two protons and two neutrons - when an atom emits an alpha particle, its atomic number reduces by 2 and its mass number reduces by 4
  • proton - positively charged + a neutron - neutral - charge decreases

Uranium-238 -> Thorium-234 + alpha particle

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What is beta decay?

  • A neutron in the nucleus turns into a proton and releases a fast-moving electron
  • Number of protons in the nucleus increases by 1 - increases positive charge of the nucleus (the atomic number)
  • Nucleus has lost a neutron and gained a proton during beta decay - the mass of the nucleus doesn't change

Carbon-4 -> Nitrogen-14 + beta particle

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What is gamma decay?

  • Gamma rays are a way of getting rid of excess energy from a nucleus - no change to the atomic mass or atomic number of the atom
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What is radioactivity?

  • Radioactive substances give out radiation from their nuclei - no matter what
  • Radiation can be measured with a Geiger-Muller tube and counter - records the count-rate
  • Radioactive decay - entirely random
  • The time it takes for the amount of radiation emitted - half-life (becquerels)
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What is half-life?

  • The half-life is the time taken for the number of radioactive nuclei in an isotope to halve
  • Some isotopes take a few hours before all unstable nuclei have decayed, whilst others last for millions of years
  • Time taken for the activity to half its initial value
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Why are there risks to using radiation?

  • Ionising radiation can enter living cells and ionise atoms within them
  • They can damage cells (cancer) or kill them off completely
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What is irradiation?

  • Objects near radioactive sources are irradiated by it (exposed to it)
  • Irradiating something does not make it radioactive
  • Reducing effects - keep sources in a lead-lined box, stand behind barriers, different room - remote-controlled arms
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What is contamination?

  • If unwanted radioactive atoms get onto or into an object
  • Contaminated atoms might then decay, releasing harmful radiation - dangerous - could get inside the body
  • Gloves, tongs and protective suits help for sources not to get trapped in the skin, under nails or breathed in
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Seriousness of irradiation and contamination

  • Cause different amounts of harm, based on radiation type


  • beta and gamma are most dangerous, they penetrate the skin
  • alpha can't penetrate the skin, blocked by a small air gap


  • alpha is most dangerous, damage in localised area, contamination = concern
  • beta, less damaging, absorbed and passes out of the body
  • gamma, least dangerous, pass straight out
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