History of the Atom

• Everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Atomic theory


  The idea that everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Democritus proposed atomic theory

  Around 500 BC
• Atomic theory was improved upon

  1800s
• John Dalton's atomic model

  Atoms are solid spheres, and different types of spheres might make up the different elements
• J.J. Thompson proposed the plum pudding model


  1897
• Plum pudding model


  • Atoms contain negatively charged particles (electrons)
  • Atoms have a general ball of positive charge with discrete electrons stuck in it
• Rutherford and his students made a big discovery


  1909
• Rutherford's experiment


  1. Fired positively charged alpha particles at a thin sheet of gold

  2. Some alpha particles were deflected to the side and a small number were even deflected back
• Rutherford's nuclear model


  Atoms have a compact nucleus containing all the positive charge, with a cloud of negative charge around it
• Rutherford's model had a flaw - the negative electrons should rush in towards the positive nucleus, causing the atom to collapse
• Niels Bohr suggested a solution to the flaw in Rutherford's model

  1913
• Bohr's model


  Electrons orbit the nucleus in shells, preventing the atom from collapsing
• Further experiments by Rutherford found that the positive charge in the nucleus is made up of small discrete particles called protons
• James Chadwick provided evidence for neutral particles in the nucleus, which we now call neutrons
• The current understanding of the atom is based on the models proposed by Rutherford and Bohr, with some small changes