History of atom

• The first major theory that had tried to explain what stuff is made out of was atomic theory, which is 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 was originally proposed by Democritus, a guy from ancient Greece, who was alive 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

  Proposed the plum pudding model, where the atom was a general ball of positive charge with discrete electrons stuck in it
• 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, proving J.J. Thompson's theory wrong
• 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 nuclear model had one important flaw, as 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 that the electrons orbited the nucleus in a similar way to how the planets orbit the sun, and that they were held in shells, which prevented 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, a guy called James Chadwick provided evidence for neutral particles in the nucleus, which we now call neutrons