P4

• Atom radius

  1x10⁻¹⁰ metres
• Subatomic constituents of an atom

  • Proton
  • Neutron
  • Electron
• Most of the mass of an atom is concentrated in the nucleus
• Arrangement of protons, neutrons and electrons in an atom

  1. Protons and neutrons in the nucleus

  2. Electrons in discrete energy levels around the nucleus
• Charge of atom nucleus

  Positive charge, due to protons
• Ways an atom's electron arrangement can change

  1. Absorbing electromagnetic radiation

  2. Emitting electromagnetic radiation
• How electron arrangement changes when atom absorbs EM radiation

  1. Electrons move further away from nucleus

  2. Electrons move to higher energy level
• How electron arrangement changes when atom emits EM radiation


  1. Electrons move closer to nucleus

  2. Electrons move to lower energy level
• Reason atom has no overall charge

  Number of protons equals number of electrons, so charges cancel
• All forms of the same element have the same number of protons
• Number of protons in an atom

  Atomic Number
• Mass number of an atom

  Total number of protons and neutrons
• Isotope

  Atom of an element with a different number of neutrons but the same number of protons
• How atoms turn into positive ions

  Lose one or more outer electrons, resulting in a positive charge
• New experimental evidence that doesn't agree with existing theory can lead to a scientific model being changed or replaced
• Plum-pudding model

  Ball of positive charge with electrons distributed evenly throughout
• Prior to discovery of electron, atom was believed to be indivisible spheres
• Experiment that led to discarding plum-pudding model

  Rutherford's alpha-scattering experiment
• Currently accepted atomic model

  Bohr nuclear model
• Conclusions of alpha-scattering experiment
• Experimental results agreeing with theoretical calculations reinforces a scientific theory
• James Chadwick's experiments proved the existence of neutrons
• Radioactive decay

  The process in which an unstable nucleus gives out radiation to become more stable
• Activity

  The rate of decay of a source of unstable nuclei
• Unit of radioactive activity

  Becquerel (Bq)
• Count-rate

  The number of radioactive decays per second for a radioactive source
• Detector used to measure count-rate

  • Geiger-Muller tube
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Alpha particle

  Two protons and two neutrons, the same as a helium nucleus
• Range of alpha particle through air

  A few centimetres (normally in the range of 2-10cm)
• What stops beta radiation

  A thin sheet of aluminium or several metres of air
• What stops gamma radiation

  Several centimetres of lead or a few metres of concrete
• Most ionising radiation

  Alpha radiation
• Least ionising radiation

  Gamma radiation
• Emission of a gamma ray does not change mass or charge
• Nature of radioactive decay

  Random, which nuclei decays and when is determined only by chance, impossible to predict
• Half-life

  The time it takes for the number of unstable nuclei in a substance to halve, or the time it takes for the count rate from a sample to fall to half its initial level
• Radioactive contamination

  The presence of unwanted radioactive nuclei on other materials
• Irradiation

  The process of exposing a material to nuclear radiation, the material does not become radioactive
• It is important for the results of studies on the effects of radiation to be published and shared with other scientists to allow the findings to be independently checked (peer review)