P7 Radioactivity

• What did John Dalton think the atom was?

  A neutral solid sphere
• What did scientists deduce about the atom's structure?

  It has a small positively charged nucleus
• What orbits the nucleus of an atom?

  Negative electrons
• What model was developed after the discovery of electrons?

  The plum pudding model
• What does the plum pudding model describe?

  • A cloud of positive charge
  • Electrons embedded within it
• What were alpha particles fired at in the experiment?

  A thin sheet of gold foil
• What were the results of the alpha scattering experiment?

  Most went straight through, some deflected
• How many alpha particles were deflected through large angles?

  1 in 10,000
• How much smaller is the nucleus compared to the atom's radius?

  10,000 times smaller
• What particles make up the nucleus?

  • Positively charged protons
  • Neutrons (neutral)
• What is the overall charge of an atom?

  Uncharged
• Why does an atom have no overall charge?

  Equal numbers of protons and electrons
• What is the basic structure of an atom?

  Nucleus with protons and neutrons
• What is the activity of a radioactive source measured in?

  Becquerel (Bq)
• What does 1 Bq represent?

  1 decay per second
• What do detectors like Geiger-Müller tubes measure?

  Count rate of decays per second
• How is the count rate after n half-lives calculated?

  Initial count rate divided by $2^n$
• What is radioactive decay?

  • Emission of nuclear radiation
  • Unstable nuclei become more stable
  • A random process
• What is ionisation?

  Knocking electrons out of atoms
• What are the types of radiation and their characteristics?

  • Alpha: highest ionising power, stopped by paper
  • Beta: high ionising power, stopped by aluminium
  • Gamma: low ionising power, stopped by lead/concrete
• What is the half-life of a radioactive source?

  Time for half the unstable nuclei to decay
• How can the half-life be determined?

  From a graph of count rate against time
• How do you find the reduction in activity after half-lives?

  Calculate activity after each half-life
• What is the mass number of an element?

  Number of protons plus neutrons
• What is the basic structure of an atom?

  The nucleus containing protons and neutrons, around which electrons orbit in fixed energy levels/shells
• What is the plum pudding model of the atom?

  A sphere of positive charge with negative electrons embedded in it
• What charges do protons, neutrons, and electrons carry?

  Protons = positive, neutrons = no charge, electrons = negative
• Why do atoms have no overall charge?

  Because they have equal numbers of positive protons and negative electrons
• What is the radius of an atom?

  Around $1\times 10^{-10}\text{ m}$
• What is ionisation?

  The process which adds or removes electrons from an atom
• What is the mass number of an element?

  The number of protons + the number of neutrons
• Which particle do atoms of the same element always have the same number of?

  Protons
• What are isotopes?

  Atoms of the same element (same number of protons) with different numbers of neutrons
• What were the two main conclusions from the alpha particle scattering experiment?

  1. Most of the mass of an atom is concentrated in the nucleus, 2) The nucleus is positively charged
• What are the three types of nuclear radiation?

  Alpha, beta, and gamma
• Which type of nuclear radiation is the most ionising?

  Alpha
• What is the range in air of alpha, beta, and gamma radiation?

  Alpha - a few cm, Beta - 1 m, Gamma - unlimited
• What are the equation symbols for alpha and beta particles?

  Alpha = $^4_2\alpha$, Beta = $^0_-1\beta$
• What is meant by the half-life of a radioactive source?

  The time taken for half the unstable nuclei to decay or the time taken for the count rate to halve
• What is radioactive contamination?

  The unwanted presence of substances containing radioactive atoms on or in other materials