atom development

• 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
• Atomic theory originally proposed by Democritus

  Around 500 BC
• It took another 2,300 years, meaning the 1800s, before anyone really improved on Democritus' ideas
• John Dalton


  Described atoms as solid spheres and suggested that different types of spheres might make up the different elements
• J.J. Thompson came up with the plum pudding model


  1897
• J.J. Thompson's experiments showed that atoms simply couldn't be solid spheres and instead that they must have contained negatively charged particles which we now know to be electrons</b>
• Ernest Rutherford and his students made a big discovery


  1909
• Rutherford's experiment


  1. Took positively charged alpha particles and fired them 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


  Proposed that the positive charge of the atom was concentrated in a compact nucleus, with the negative charge existing in a cloud around this central nucleus
• Rutherford's model had one important flaw - there didn't seem to be anything stopping the cloud of negative electrons from rushing in towards the positive nucleus, meaning the atom should just automatically collapse
• Niels Bohr suggested a solution to the flaw in Rutherford's model


  1913
• Bohr's model


  Suggested the electrons orbited the nucleus in a similar way to how the planets orbit the sun, and that they were held in shells
• Bohr's model of the atom, with the electrons orbiting the nucleus, is what prevents the atom from collapsing
• Further experiments by Rutherford found that the positive charge in the nucleus is actually made up of small discrete particles which we now know as protons
• A short while later, James Chadwick provided evidence for neutral particles in the nucleus which we now call neutrons
• The current model of the atom is pretty much the same as Bohr's model, with just a few small changes based on further experiments