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440 BCE

Democritus first proposed that everything in the world was made up of tiny particles, which varied in size and shape and were surrounded by empty space.  He called the particles "Atomos" which is Greek for indivisible.

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Aristotle, opposed Democritus with a different theory at the time, as he thought that Democritus was stupid for saying such a thing.  Aristotle thought that everything could be brought back to being made from four main elements;  Earth, Wind, Fire, and Water.

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John Dalton (1766-1844) wanted to challenge Aristoterian theory.  Dalton showed that common substances always broke down into the same elements in the same proportions.  He concluded that various compounds were combinations of atoms of different elements, each of a particular size and mass and could neither be created nor destroyed.

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J.J Thomson (1856-1940) discovered the electron.  He then provided a model of what he thought an atom was which he showed as spheres of positively charged matter, filled with negatively charged electrons.  This then led him to win a Nobel prize in1906.

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One of J.J Thomson's students (Ernest Rutherford AKA "the father of the nuclear age, 1871-1937) Discovered the Nucleus of the Atom.  In a test using positively charged alpha particles and shooting them at a sheet of gold foil,  he discovered something that should not have happened if Thomson's model was correct.  Because some particles were reflected he made a new model which had a nucleus in the centre with empty space outside of it, and a few electrons surrounding the nucleus and empty space.

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Another one of Thomson's students (Neils Bohr, 1885-1962) expanded on Rutherland's model by referring to earlier work by Max Plack and Albert Einstein.  Bohr stipulated that electrons orbited around the nucleus at fixed energies and distances, being able to jump from one level to another, but not exist in the space between.  He later discovered that electrons did not only act as particles but also as waves, not being confined to a particular point in space.  Werner Heisenberg (1901-1976)  then showed that it was impossible to determine the exact speed and location of electrons as they moved around an atom, Which then lead to the current quantum model of the atom.

Below; Neils Bohr

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Below; Werner Heisenburg