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Democritus expanded this theory to state that matter was composed of small particles called "atoms" that could be divided no further. These atoms were all composed of the same primary matter with the only differences between them being their size, shape, mass, positions and arrangement.

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Aristotle thought there was just water, air, earth and fire.

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John Dalton believe all elements are made up of atoms. It is impossible to divide or destroy an atom. He named his model the Billiard Ball Model.

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J.J Thomson’s model was known as the "Plum Pudding Model” (or "Raisin Bread Model.") As each atom was a sphere filled with a positively charged fluid, known as the “pudding”. Scattered in this fluid were negatively charged electrons, these were the “plums” in the pudding.

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Lord Ernest Rutherford
Rutherford proposed that atoms consisted of a small dense center filled with positive charges.  He named this center the nucleus. He then said that negatively charged electrons were scattered  surrounding this dense, positively charged center.  He stated these negatively charged electrons were held in orbit by the positively charged nucleus, due to the electrical forces.

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Bohr’s atom like Rutherford’s contained a dense, positive nucleus. He expanded on Rutherford’s ideas, turning his attention to describing the electron. He stated they moved in fixed, circular orbits (or energy levels) around the nucleus, these called electron shells. These shells were at set distances from the nucleus and were the same for all atoms. He stated they became larger the further away they are from the nucleus, and that electrons furthest from the nucleus have higher energy.