Atomic structure

• Daltons model of the atom

  Solid sphere that cannot be divided into smaller parts

  Did not include protons, neutrons or electrons
• Plum pudding model

  Cloud of positive charge with negative electrons embedded in it

  Protons and neutrons had not yet been discovered
• In what order were the sub-atomic particles discovered?

  Electrons -> Protons -> Neutrons
• Neutron: particle with no charge found within an atom
• Proton: positively charged particle found within an atom
• Electron: negatively charged particle found within an atom
• Alpha scattering experiment

  1. Scientists fired alpha particles at a gold sheet only a few atoms thick
  2. Expected it to just go through, but instead they found that some went through, but some were scattered
  3. Concluded that the positive charge and mass of an atom must be held in the centre (the nucleus)
• Nuclear model

  Electrons orbit the nucleus, but not at set distances

  Mass of atom was concentrated in the charged nucleus
• Bohr model

  Calculated the electrons must orbit at set distances

  Orbits called shells or energy levels
• Order of different models of atoms

  Daltons model

  Plum pudding model

  Nuclear model

  Bohr model
• How can Electron gain or lose energy?

  Gain energy by absorbing electromagnetic radiation -> move to a higher energy level

  Lose energy by emitting electromagnetic radiation -> move to a lower energy level
• Different parts of the element symbols

  Top number: mass number -> number of protons + neutrons

  Bottom number: atomic number -> number of protons
• Isotopes: atoms of the same element with the same number of protons, but different number of neutrons
• Ionisation

  Atoms become charged ions, by gaining or losing electrons

  Positive ion: Uncharged atom loses electron(s)

  Negative ion: Uncharged atom gains electron(s)
• The atom

  Radius of about 1x10^-10 meters

  Uncharged overall

  Equal number of protons and electrons
• The nucleus

  Radius about 1x10^-15 (10,000x smaller than atom)

  Contains protons and neutrons

  Where most of the mass is concentrated
• Radioactive decay: unstable nuclei emit nuclear radiation so they become more stable, it is a random process
• Alpha particles, What, Symbol, Change in nucleus, Ionising power, Range in air, Stopped by

  What: 2 protons and 2 neutrons

  Symbol: α

  Change in nucleus: Loses 2 protons and 2 neutrons

  Ionising power: Highest

  Range in air: Few centimetres

  Stopped by: Sheet of paper
• Beta particle, What, Symbol, Change in nucleus, Ionising power, Range in air, Stopped by

  What: Fast moving electron

  Symbol: β

  Change in nucleus: Neutron changes into a proton and an electron

  Ionising power: High

  Range in air: Around 1m

  Stopped by: Few mm of aluminum
• Gamma radiation, What, Symbol: Change in nucleus, Ionising power, Range in air, Stopped by

  What: Short wavelength, high frequency, electromagnetic radiation

  Symbol: γ

  Change in nucleus: Some energy is transferred away

  Ionising power: Low

  Range in air: Unlimited

  Stopped by: Several cm of thick lead or metres of concrete
• Activity of a radioactive source: Rate of decay of an unstable nucleus, measured in Bq (1 Bq = 1 decay per second)
• Count rate of a radioactive source: Number of decays detected per second (by a detector such as a Gieger-Muller tube)
• Half life of a radioactive source

  1. The time for half the number of unstable nuclei in a sample to decay
  2. The time for the count rate or activity of a source to half
• When is alpha, beta, and gamma radiation dangerousand why

  Alpha: Inside the body as it affects all surrounding tissue

  Beta and gamma: Outside and inside as they can penetrate tissues
• Irradiation

  When an object is exposed to nuclear radiation

  Prevented by shielding, removing or moving away from the source
• Contamination

  When atoms of radioactive material are on or in an object

  Object remains exposed to radiation as long as its contaminated

  Contamination is very hard to remove