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460 B.C. - 370 B.C.
- Democritus proposed an atomic theory.
- According to Democritus if you tried to cut matter into the smallest pieces possible, you would eventually get a very small particle that is indestructible and could not be cut. He called these particles atoms and believed that they are the basic building blocks of all matter.
- Democritus did not have any technology available to him and did not conduct any experiments to test his ideas, so he did not have any proof that atoms existed and no evidence to back up his claims.
- There were no previous model, this is the first.
- He didn't know that there were smaller particles within the atom.

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1766 - 1844
- Dalton formulated the first modern  description of it as the fundamental building block of chemical structures.
- Dalton's fascination with gases gradually led him to formally assert that every form of matter was also made up of small individual particles.
- Dalton also explain that there were proton, electron, and neutron within the atom and the previous atomic model.
- In the later years it was improved upon by other scientists.

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1856 - 1940
His model showed that all atoms contained tiny negatively charged subatomic particles called corpuscles later known as electrons. This was called the Plum Pudding model. The electrons were believed to be inside of the nucleus floating on a positively charged soup substance. To create this model, the discovery of electrons, the atom, and a positive substance. There were pros as opposed to the previous model because the concept of the nucleus and electrons are now known. There were limitations because his model failed to explain how the positive charge holds the electrons, the atom's stability, and the nucleus was not talked about much. Neutrons were also not included and there was no structure.

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1885-1962
According to classical theory, comparable amounts of light energy should be produced at all frequencies. This is not only contrary to observation but also implies the absurd result that the total energy radiated by a heated body should be infinite.  Poor spectral predictions are obtained when larger atoms are in question. It cannot predict the relative intensities of spectral lines.

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1871 - 1937
The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. The model  was unable to explain the stability of an atom.

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1887 - 1961
A powerful model of the atom was developed by Erwin Schrodinger in 1926. Schrodinger's model allowed the electron to occupy three-dimensional space. It therefore required three coordinates, or three quantum numbers, to describe the orbitals in which electrons can be found. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects.

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1891 - 1974
James discovered the charge of the neutron.  He discovered that a neutron was a particle with no electric charge so it was neutral, and that those along with protons made up the nucleus. The electrons were still known to be outside the nucleus. For this model to be possible, the discovery of the atom, protons, the nucleus, electrons, and neutrons had to be made along with their charges. There were more pros as opposed to the previous model because it now had neutrons in the nucleus with the protons. There were limitations when it came to how the electrons did not attract to the protons because opposites attract. Without this being explained, many would think the atom will become unstable and implode because of the electrons and protons attraction.