P1.1 - The Particle Model

• Democritus thought that if you cut wood enough times, you would eventually not be able to cut it any further, and what you would be left with was atoms.
• Dalton's model showed atoms as small indestructible spheres. He thought that:

  • All atoms of the same element are the same.
  • Atoms of one element are different to atoms of another.
• Thompson discovered electrons with cathode rays, which bent towards an oppositely charged magnet. He thought that:

  • An atom contains negative electrons.
  • Overall, atoms have no charge (they are neutral).
• With the plum-pudding model, Thompson showed his theory that electrons were inside a positive mass.
• Rutherford found that some materials emit particles, which he called alpha particles. He shot the positive particles at a piece of gold foil and found that some bounced back.
• Rutherford found that:

  • An atom has a positively charged nucleus.
  • Most of an atom's mass derives from the nucleus.
  • Electrons orbit the nucleus.
• The problem with Rutherford's model is that there was no explanation for why the electrons wouldn't spiral into the nucleus.
• Bohr suggested that electrons occupied energy levels / electron shells around a nucleus.
• An atom's diameter is 1 x 10^-10m.