The particle model

• Everything is made of matter. Matter is made of atoms. Atoms are too small to see with the human eye
• For many years there were no developments on the idea of atoms and no way of measuring them
• In 1870s, John Dalton did experiments to measure how elements combine and that this would show atoms make up everything.
• Daltons model of an atoms depicts a very small indestructible sphere. All atoms in an element are the same and the atoms in each element are different form the atoms in all other elements
• In 1897, JJ Thomson discovered cathode rays are made of particles. He found a particle that came from inside an atom that we now know as an electron. He worked out that the charge on this was negative.
• Thomson thought that atoms contained negative electrons and that overall the charge was neutral
• In 1904, Thomson suggested a new model that he called the plum pudding model.
• In the plum pudding model, the atom was though to consist of a positive mass with negative electrons embedded in it. He didn't know what the positive mass was made of.
• In 1899, Rutherford discovered that some materials emit particles. Called alpha particles, these have a positive charge. He decided to fire alpha particles at a piece of gold foil.
• In Rutherfords experiment, most particles went straight through the sheet indicating empty space. Some particles were deflected indicating a small mass. Few particles were reflected indicating a centrally concentrated mass
• Rutherford suggested that's an atom is made of a tiny positively charged nucleus with electrons orbitting around the outside. This is called Rutherfords model
• Rutherford says almost all the mass is in the nucleus. This was later discovered as being made of protons and neutrons
• Rutherford had a problem. Electrons would spiral until they hit the nucleus as they attracted to a positive charge
• In 1913, Neils Bohr suggested that electrons can only move in fixed orbits called electron shells
• Molecules are collections of atoms, strongly bonded together. A Molecules is 100 atoms × 10‐¹⁰ m/ per atom
• Nucleus

  • Contains protons and neutrons
• Electron


  • Moves around the nucleus
• Subatomic particles

  • Proton
  • Neutron
  • Electron
• The Bohr model of the atom is used by scientists
• The diagram is not to scale