The model 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
• Plum pudding model


  • An atom is a sphere of positive charge, with negatively charged electrons in it
  • Consistent with evidence that solids cannot be squashed, so atoms must be solid throughout
  • 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
• Atom


  The building block of matter
• Atom


  • A collection of smaller particles
  • Not a single particle about 1 × 10‾¹⁰ m across
• 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
• 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
• Electron


  Subatomic particle with a negative charge and 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)
  • The electrons jump back down to a lower level, releasing light of definite frequencies
• Proton


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


  Neutral particle the same size as a proton, keeping the nucleus stable and making 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