atomic structure

• atoms are very small. They have a radius of 1 x 10 ^ -10 metres
• the basic structure of an atom is a positively charged nucleus composed of positive protons and neutral neutrons surrounded by negatively charged electrons
• the radius of the nucleus is 1 x 10 ^ -14 metres
• electrons are arranged at different energy levels (different distances from the nucleus). The arrangement may change with the absorption of electromagnetic radiation (electrons move to a higher energy level, further away from the nucleus) or the emission of electromagnetic radiation (electrons move closer to the nucleus, at a lower energy level)
• radioactive decay is the process by which unstable nuclei give out radiation to become more stable. the process is random.
• activity is the rate at which a source of unstable nuclei decays
• the count rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube)
• nuclear radiation that can be emitted is:

  • an alpha particle - consists of two protons and two neutrons, the same as a helium nucleus
  • a beta particle - a high energy electron ejected from the nucleus as a neutron turns into a proton
  • gamma rays - electromagnetic radiation from the nucleus
  • a neutron
• rutherford's alpha scattering practical --

  • fired alpha particles at a sheet of gold foil in a vacuum
  • most of the particles went straight through, which proved the atom was mostly empty space
  • some particles were deflected slightly, which proved the nucleus was positively charged
  • few particles were deflected at an angle greater than 90 degrees, which proved the nucleus contained most of the mass, and electrons surrounded the nucleus
• nuclei are unstable due to an imbalance of forces within the nucleus
• alpha particle:

  • highly ionising
  • weak penetrating power
  • can travel 5cm in air
  • stopped by paper and skin
• beta particle:

  • medium ionising power
  • medium penetrating power
  • can travel around 50cm in air
  • stopped by thin sheet of aluminium
• the half life of a radioactive isotope is the time taken for half the nuclei in a sample to decay
• radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials
• irradiation is the process of exposing an object to nuclear radiation. the irradiated object does not become radioactive
• materials become radioactive if the material has been contaminated.
• contamination of the body causes cancer as cells are ionised, and then those cells divide uncontrollably, causing tumors
• background radiation is around us all the time. It comes from:

  • natural sources such as rocks and cosmic rays from space
  • man-made sources such as the fall-out from nuclear weapons testing and nuclear accidents
• radiation dose is measured in sieverts (Sv)
• fission:

  • splitting of a large and unstable nucleus (uranium or plutonium)
  • spontaneous fission is rare
  • usually, for fission to occur, the unstable nucleus must first absorb a neutron
  • the nucleus undergoing fission splits into two smaller nuclei (roughly equal in size), emits two or three neutrons (can start a chain reaction) and gamma rays
  • energy is released by the fission reaction
  • all the fission products have kinetic energy
  • the chain reaction is controlled in a nuclear reactor to control the energy released
  • nuclear weapon explosions are caused by an uncontrolled chain reaction
• fusion:

  • nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing lots of energy
  • the sum of the masses of the two lighter nuclei is more than the mass of the heavier nucleus because some of the mass is converted into energy (released as radiation)
  • the sun is a natural nuclear fusion reactor
  • fusion would be a more efficient way to produce energy than fission however no design has been produced that could accomplish positive net energy on Earth