P7 radioactivity

• John Dalton thought the atom was a neutral solid sphere you cannot divide into smaller parts.
• The discovery of negatively charged electrons led to the plum pudding model
• a cloud of positive charge with electrons embedded in it
• Positively charged alpha particles were fired at a thin sheet of gold foil
• To explain the results, scientists deduced that there is a small positively charged nucleus at the centre of the atom where most of the mass is concentrated.
• The negative electrons orbit the nucleus
• Bohr suggested the electrons orbit at specific distances called energy levels.
• The nucleus, which is 10 000 times smaller than the radius of the atom, consists of two particles:

  • positively charged protons

  • neutrons which are neutral

  An atom is uncharged overall and has equal numbers of protons and electrons.
• Isotopes are atoms of the same element, with the same number of protons but a different numbers of neutrons.
• atomic number

  number of protons
• mass number

  number of protons + neutrons
• Radioactive decay is when nuclear radiation is emitted by unstable atomic nuclei so that they become more stable.
• ionisation can knock electrons off atoms
• alpha particle (two protons and two neutrons)
• beta particle (fast‑moving electron)
• gamma radiation (short‑wavelength, high‑ frequency EM radiation)
• The activity of a radioactive source is the rate of decay of an unstable nucleus, measured in becquerel (Bq
• Detectors (e.g., Geiger‑Müller tubes) record a count rate (number of decays detected per second)
• The half‑life of a radioactive source is the time 


  • for half the number of unstable nuclei in a sample to decay

  • for the count rate or activity of a source to halve.
• To find the reduction in activity after a given number of half‑lives:

  1 calculate the activity after each half‑life

  2 subtract the final activity from the original activity.
• Describe the basic structure of an atom.

  nucleus containing protons and neutrons, around which electrons orbit in fixed energy levels/shells
• What is the radius of an atom?

  around 1×10−10 m
• What is ionisation?

  process which adds or removes electrons from an atom
• What are isotopes?

  atoms of the same element (same number of protons) with different numbers of neutrons
• What is the range in air of alpha, beta, and gamma radiation?

  a few cm, 1m, and unlimited