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Democritus hypothesized that all matter is made of extremely small, unbreakable circular units, that we today call atoms. The inspiration behind his theory was to answer the question 'How finely can one go cutting up matter?' At the time he had not claimed any solid information due to lack of experiments and research. Which, means no science was applied to answer the question, only curiosity and random ideas.
Democritus' atomic theory:

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Aristotle was a Greek philosopher who helped extend the theory of the atom. His idea was that all matter consisted of 4 elements; fire, water, earth and air. He thought that these 4 elements were everywhere, however in very small particles. Considering Aristotle was a philosopher, he didn't perform any experiments and had no scientific evidence behind the theory.
This is Aristotle's atomic theory

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Dalton's theory was the first to describe atoms and their properties. He also stated that all matter is made up of atoms that are indivisible. His theory revolved around the law of conservation of mass and the law of constant composition. He began the idea that the atoms in liquids are identical in mass and properties, and the same thing goes for any given element.

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In 1897, Thomson discovered the electron during his research. He used a chathode ray to determine the charge to mass ratio of an atom.

Thompson discovered that atoms have positively charged cores, with negatively charged particles within them.

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Millikan calculated the charge of a single elctron in 1908. This discovery allowed us to understand greater information about the structure of the atom and the atomic theory.
Millikan used an oil drop to calculate the charge of an atom.

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Niels introduced the idea that an atom is a positively charged nucleus that is orbitted by electrons. He calculated possible energy levels for the atom to orbit.

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Using mathematical equations, erwin calculated the likelihood of an electron being in a certain position.

This model is called the quantum mechanical model of the atom.

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- http://timelineoftheatom.blogspot.ae/p/2.html
- https://www.barcodesinc.com/articles/timeline-on-atomic-structure.htm
- https://the-history-of-the-atom.wikispaces.com/Aristotle
- http://atomictimeline.net/index.php
- http://www.abcte.org/files/previews/chemistry/s1_p5.html
- http://study.com/academy/lesson/robert-millikan-biography-atomic-theory-oil-drop-experiment.html
- http://www.abcte.org/files/previews/chemistry/s1_p6.html