P6: RADIOACTIVITY

• Atom


  Radius of 1x10^-10m, very tiny and dense, with electrons orbiting its nucleus

  
• Nucleus


  Contains protons and neutrons, packed very close together in a small region of space, overall positive charge

  
• Number of protons


  Determines the element

  
• Radius of the nucleus is about 10,000 times smaller than the atom
• All of the atom's mass is concentrated in the nucleus

  
• Electron energy levels


  Electrons orbit the nucleus at particular fixed distances, called energy levels, a certain number can occupy each level, higher energy levels have higher potential energy

  
• Electron energy level changes

  1. Absorbing electromagnetic radiation (excitation)

  2. Emitting electromagnetic radiation (de-excitation)

  
• Ionisation


  Atom gains or loses an electron from the outer energy level, forming a charged particle (ion)
• Ways positive ionisation can occur


  • Rubbing objects together (friction)
  • Electrons absorbing electromagnetic radiation
  • Chemical reactions
• Negative ionisation most commonly occurs during chemical reactions
• Absorption of electromagnetic radiation

  Electrons move to higher energy levels

  
• Dark coloured objects


  Good absorbers of radiation, do not reflect energy that hits them
• Emission of electromagnetic radiation

  Electron moves back down to original energy level, emitting a wave of electromagnetic radiation

  
• All colours in the visible spectrum are produced by electrons moving down energy levels and emitting electromagnetic radiation

  
• Protons


  Positively charged particles with relative atomic mass of 1, found in the nucleus

  
• Neutrons


  Neutral particles with relative atomic mass of 1, found in the nucleus

  
• Electrons


  Negatively charged particles with relative atomic mass of 1/2000, orbit the nucleus

  
• The total charge of an atom is 0, because there are the same number of electrons as protons

  
• Atomic number


  Number of protons in an atom, also shows the number of electrons

  
• Mass number


  Total number of protons and neutrons in a nucleus

  
• Isotopes


  Atoms of the same element with the same number of protons but different number of neutrons

  
• Unstable nuclei


  Imbalance in forces in the nucleus, due to large size or too many/too few neutrons

  
• Radioactive decay


  Unstable nuclei emit radiation to become more stable, reducing the overall energy of the nucleus

  
• Radioactive decay is a random process, unpredictable when a particular nucleus will decay

  
• Types of radioactive decay

  • Alpha
  • Beta
  • Gamma

  
• Alpha radiation


  Helium nucleus with 2 neutrons and 2 protons, charge of 2+

  
• Beta radiation


  High energy, fast moving electrons produced in the nucleus when a neutron changes into a proton and an electron, charge of 1-

  
• Gamma radiation


  Electromagnetic waves with the highest energy, no charge

  
• Geiger-Muller tube


  Device used to detect radiation, creates ions when radiation passes through it

  
• Geiger-Muller counter


  Counts the ions created in the Geiger-Muller tube
• Count-rate


  Number of decays recorded each second by a detector
• Alpha decay


  Alpha particle emitted, new element formed, mass number decreases by 4, atomic number decreases by 2
• Beta decay

  Neutron changes into proton and electron, electron emitted, new element formed, atomic number increases by 1, mass number stays the same

  
• Gamma decay


  Gamma ray emitted, no change in mass, atomic number or mass number
• Alpha, beta, gamma radiation


  • Alpha most ionising but least penetrative, gamma least ionising but most penetrative

  
• Range in power


  More ionising radiation reacts sooner with air, strongly ionising radiation has shortest range

  
• Half-life


  Time for radioactive substance to decrease by half, or for activity to halve
• Calculating radioactive decay

  Measure initial activity, determine half-life, measure how activity changes over time
• Contamination

  Unwanted presence of radioactive materials on other materials, often due to radiation leak
• Irradiation

  Exposing a material to alpha, beta or gamma radiation, does not make it radioactive but can kill living cells, used for sterilisation