Physics 2B: Atoms and Radiation

Physics 2B: Atoms and Radiation

1. History of Atomic Structure

   • 1804 - John Dalton thought matter was made up of unbreakable 'tiny spheres'.
   • 100 years later J. J. Thomson discovered electrons could be removed - proved Dalton wrong.
   • Thomson thought atoms were spheres of positive charge with negative electrons in them like a plum pudding - made the Plum Pudding model.
   • 1909 - Ernest Rutherford and Marsden fired a beam of alpha particles at gold foil.
   • Expected positive alpha's to be slightly deflected by the electrons - actually most went straight through with the odd one coming straight back.
   • Realised most of the mass was in the centre - the nucleus - and that it must have a positive charge. Also showed most of an atom is empty space.
   • Rutherford came up with the Nuclear Model (the one we use today).

2. Atomic Structure

   • The radius of an atom is 0.1 nanometres.
   • The radius of a nucleus is 10, 000 smaller but contains almost all the mass.
   • Electrons are at different distances from the nucleus - called energy levels.
   • Atom absorbs electromagnetic radiation, electrons can move to a higher level. If emitted, can move back down.

3. Isotopes

   • Same number of protons, different number of neutrons.
     • Same atomic number, different mass number.
   • Isotopes tend to be radioactive, meaning they decay into other elements and give out radiation.

4. Radioactivity Basics

   • Radiation is given out from the nucleus, and is entirely random.
     • Unaffected by physical conditions like temp, chemical bonding etc.
   • Background Radiation
     • 50% comes from radon gas in the ground.
     • Most from other natural sources like cosmic rays, food, building materials and rocks.
     • Also from man-made sources like x-rays, nuclear accidents (Chernobyl) and nuclear weapons testing (1%).

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