Atomic structure

• Why do unstable nuclei give out radiation?

  Unstable nuclei undergo decay to become more stable.
• What is the name of the process in which an unstable nucleus gives out radiation to become more stable?

  Radioactive decay.
• Define the activity of an unstable nucleus.

  Activity is the rate of decay of a source of unstable nuclei.
• What is the unit of radioactive activity?

  Becquerel (Bq).
• What is count-rate?

  The number of radioactive decays per second for a radioactive source.
• Give an example of a detector that may be used to measure count-rate.

  Geiger-Muller tube.
• State four types of nuclear radiation.

  1. Alpha particles
  2. Beta particles
  3. Gamma rays
  4. Neutrons
• What are the constituents of an alpha particle?

  Two protons and two neutrons.
• What is the range of an alpha particle through air?

  A few centimetres (normally in the range of 2-10 cm).
• What will stop beta radiation from passing through a point?

  A thin sheet of aluminium or several metres of air.
• What will stop gamma radiation from passing through a point?

  Several centimetres of lead or a few metres of concrete.
• Which type of radiation is most ionising?

  Alpha radiation.
• Which type of radiation is least ionising?

  Gamma radiation.
• State any changes to mass or charge that occur due to the emission of a gamma ray.

  Both mass and charge remain unchanged.
• Describe the nature of radioactive decay.

  • Random
  • Which nuclei decays and when is determined only by chance
  • It is impossible to predict which nuclei will decay and when
• Define the half-life of a radioactive isotope.

  The time it takes for the number of unstable nuclei in a substance to halve.
• What is radioactive contamination?

  The presence of unwanted radioactive nuclei on other materials.
• What is irradiation?

  The process of exposing a material to nuclear radiation without making it radioactive.
• Why is it 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
  • This is known as peer review
• Give an approximation for the radius of an atom.

  1 x 10⁻¹⁰ metres.
• What are the three subatomic constituents of an atom?

  Proton, neutron, electron.
• Where is most of the mass of an atom concentrated?

  In the nucleus.
• Approximately what proportion of the total radius of an atom is the radius of the nucleus?

  1/10,000.
• Describe the arrangement of protons, neutrons and electrons in an atom.

  • Protons and neutrons are found in the atom’s nucleus
  • Electrons are found in discrete energy levels around the nucleus
• What proportion of the total radius of an atom is the radius of the nucleus?

  Approximately 1/10,000.
• Describe the arrangement of protons, neutrons, and electrons in an atom.

  • Protons and neutrons are found in the nucleus.
  • Electrons are found in discrete energy levels around the nucleus.
• What type of charge does the nucleus of an atom have and why?

  The nucleus has a positive charge because it contains protons.
• Give two ways that an atom’s electron arrangement can be changed.

  1. Absorbing electromagnetic radiation.
  2. Emitting electromagnetic radiation.
• How does an atom’s electron arrangement change when it absorbs EM radiation?

  Electrons move further away from the nucleus to a higher energy level.
• How does an atom’s electron arrangement change when it emits EM radiation?

  Electrons move closer to the nucleus to a lower energy level.
• How does the ratio of electrons to protons in an atom result in the atom having no overall charge?

  The number of protons equals the number of electrons, canceling their charges.
• What do all forms of the same element have in common?

  They all have the same number of protons.
• What is the name given to the number of protons in an atom?

  Atomic Number.
• What is an atom’s mass number?

  The total number of protons and neutrons in the atom.
• What is an isotope of an atom?

  An atom of an element that has a different number of neutrons but the same number of protons.
• How do atoms turn into positive ions?

  They lose one or more of their outer electrons, resulting in a positive charge.
• What may lead to a scientific model being changed or replaced?

  The discovery of new experimental evidence that doesn’t agree with the existing theory.
• How did the plum-pudding model describe the atom?

  A ball of positive charge with negatively charged electrons distributed evenly throughout it.
• What was believed about the atom prior to the discovery of the electron?

  The atom was believed to be indivisible.
• Which experiment led to the plum-pudding model being discarded?

  Rutherford’s alpha-scattering experiment.