modelf of the atom

• The idea of the atom as the building block of matter has developed over time
• What was thought of as a single particle about 1 × 10‾¹⁰ m across is now known to be a collection of smaller particles
• Atom


  The smallest part of an element that can exist
• Early ideas of matter in ancient Greece

  • Matter was made up of combinations of four elements: earth, air, fire and water
  • A small number of Greeks believed matter could be cut into smaller and smaller pieces until it ended up as a piece that couldn't be cut anymore
• Atom

  The building block of matter
• Atom

  • Previously thought to be a single particle about 1 × 10‾¹⁰ m across
  • Now known to be a collection of smaller particles
• Ernest Rutherford did an experiment to test the plum pudding model


  1905
• Alpha particle

  Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus)
• Vacuum

  A volume that contains no matter
• Alpha particles are a form of nuclear radiation with a large positive charge
• Atomos

  Uncuttable (the origin of the word 'atom')
• The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else
• Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking
• It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it
• If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected
• Observations from Rutherford's experiment

  • Most of the alpha particles did pass straight through the foil
  • A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil
  • A very small number of alpha particles came straight back off the foil
• Rutherford's conclusions

  • The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space
  • A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges
  • The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny
• Plum pudding model


  The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons in it
• Nuclear atom

  A small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom
• The nucleus was calculated to be about 1/10,000th the size of the atom
• The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford
• J J Thomson proposed the plum pudding model after discovering the electron in 1897
• The plum pudding model was consistent with the evidence available at the time: solids cannot be squashed, therefore the atoms which make them up must be solid throughout
• Atom

  The building block of matter
• Atom

  • Previously thought to be a single particle about 1 × 10‾¹⁰ m across
  • Now known to be a collection of smaller particles
• Nucleus

  The central part of an atom, containing protons and neutrons, and having most of the atom's mass
• Rubbing two solids together often results in static charge, so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide
• Electron

  A subatomic particle with a negative charge and a negligible mass relative to protons and neutrons
• Bohr's model of the atom

  1. Electrons orbit the nucleus in different energy levels or at specific distances from the nucleus

  2. Particular chemicals burn with certain-coloured flames because the pattern of energy released by electrons in the chemical reaction is the same for every single atom of that element

  3. Electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom
• When atoms absorb energy

  • The electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus)
  • In time, they jump back down to a lower level releasing light of definite frequencies
• Proton

  The nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass
• Neutron

  A neutral particle the same size as a proton that keeps the nucleus stable and makes up the mass
• James Chadwick proved the existence of the neutron


  1932
• Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil to prove the existence of the neutron