Atomic structure

• John Dalton theorised that atoms were solid spheres that could not be broken up.
• J.J. Thompson developed the 'plum pudding' model of the atom.
• Rutherford carried out the alpha particle scattering experiment - this showed that an atom's mass was concentrated in its centre.
• Niels Bohr proposed that electrons orbit the nucleus in shells.
• James Chadwick discovered the neutron in 1932.
• Unstable isotopes tend to decay into other elements, and give out radiation as they try to become more stable. This process is called radioactive decay.
• The three types of radiation are alpha, beta and gamma.
• Ionising radiation knocks electrons off atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this.
• Alpha radiation is when an alpha particle is emitted from the nucleus. An alpha particle is two neutrons and two protons (a helium nucleus).
• Alpha radiation does not penetrate very far (they can be stopped by paper), but they are very ionising.
• Alpha radiation is used in smoke detectors.
• A beta particle is a fast moving electron released by the nucleus.
• Beta particles are moderately ionising and penetrating. They can be stopped by a 5mm sheet of aluminium.
• For every beta particle emitted, a neutron in the nucleus turns into a proton.
• Gamma rays are waves of electromagnetic radiation released by the nucleus.
• Gamma rays penetrate far into materials and travel a long distance in air.
• Gamma rays are weakly ionising as they tend to pass through (rather than collide) with atoms.
• Nuclear equations take the form of:

  atom before decay --> atom after decay + radiation emitted
• Alpha decay decreases the charge and mass of the nucleus.
• Beta decay increases the charge of the nucleus.
• Gamma rays don't change the charge or mass of the nucleus.
• Radioactive decay is entirely random.
• The half-life is the time taken for the number of radioactive nuclei in an isotope to halve.
• Radioactive activity is measured in becquerels (Bq).
• Background radiation is the low-level radiation around us all the time. It should be subtracted from any results to avoid systematic error.
• Background radiation comes from:

  • naturally occurring unstable isotopes in food, air and rocks
  • cosmic rays from space
  • human activity like fallout from nuclear explosions or waste
• Being irradiated means being exposed to a radioactive source. It does not make something radioactive.
• If radioactive atoms get on or into an object, then the object is contaminated. This could cause you harm if you become contaminated.
• Alpha radiation is the least dangerous radiation outside of the body because is cannot penetrate the skin.
• High radiation doses can kill cells completely, causing radiation sickness.
• Radiation can cause minor damage to cells, giving rise to mutant cells that divide uncontrollably: cancer.
• Gamma radiation sources can be used in medical tracers. This is because they pass out of the body without causing much ionisation.
• Gamma rays can also be used to kill cancer cells by aiming high doses directly at the cancerous cells.
• Radiation therapy can damage some healthy cells, making the patient feel tired and weak.
• Nuclear fission is a reaction that releases high amounts of energy from splitting large unstable atoms.
• For fission to occur, the nucleus needs to absorb a neutron before it splits. When it does, it forms two lighter elements that are roughly the same size, and two or three neutrons. These neutrons can then be absorbed by other nuclei, causing a chain reaction.
• The energy not transferred to the kinetic stores of the products during fission is released as gamma rays.
• The amount of energy produced by fission in a nuclear reactor is controlled by 'control rods', which absorb neutrons and slow down the chain reaction.
• Uncontrolled fission reactions lead to an explosion - this is how nuclear weapons work.
• Fusion is joining two small nuclei to form a larger nucleus. Example: hydrogen fusing to produce helium.