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In 403 BC Democritus was the first to propose the idea of atoms. He believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped. The shape and sizes of atoms were determined by the various properties of matter.

Sources:
https://www.britannica.com/science/atom/Development-of-atomic-theory
https://sites.google.com/site/mrjhatomichistory/democritus

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In 1803 John Dalton proposed the solid sphere model or also known as the bowling ball model. Dalton’s atomic theory has 4 parts. The first part of the theory states that all matter is made of atoms. The second part of the theory states that all atoms of a given element are identical in mass and properties. The third part says compounds are combinations of two or more different types of atoms. The fourth part states that a chemical reaction is a rearrangement of atoms. His theory was the first complete attempt to describe all matter in terms of atoms and their properties.

Sources:
https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/history-of-atomic-structure/a/daltons-atomic-theory-version-2
http://what-when-how.com/scientists/dalton-john-1766-1844-english-physicist-chemist-scientist/

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In 1897, J.J. Thomson discovered the electron by using the cathode ray experiment. After creating this experiment he realized that the model of an atom did not account for negative and positive charges. That is why he proposed the plum pudding model. The raisins represented the negative charges and the dough represented the positive charges.

Sources:
http://www.abcte.org/files/previews/chemistry/s1_p5.html
https://commons.wikimedia.org/wiki/File:Portrait_of_Joseph_John_Thomson.jpg

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In 1911 Ernest Rutherford performed a series of experiments the most notable of which is called the gold foil experiment. With this experiment, he discovered that most particles passed through the gold foil as if it were empty space some of them diverted from its course. Rutherford proposed an atomic model also known as the “Rutherford nuclear atom” and the “Rutherford Planetary Model”. Rutherford’s model compared to has more empty space Dalton’s and Thompson's models whereas in the earlier models atoms were considered solid.

Sources:
https://www.primaryconnections.org.au/themes/custom/connections/assets/SBR/data/Chem/sub/atom2/atom2.htm#n.
https://en.wikiquote.org/wiki/Ernest_Rutherford

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In 1913 Bohr proposed his quantized shell model of the atom. This model explains how electrons can have stable orbits around the nucleus. This was made as an improvement from Rutherford’s model because the model he proposed was unstable. To fix the stability issue Bohr proposed that the electrons would move in orbits.

Sources:
http://abyss.uoregon.edu/~js/glossary/bohr_atom.html
https://en.wikipedia.org/wiki/Niels_Bohr

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In 1926 Erwin Schrödinger an Austrian physicist took the Bohr atomic model and advanced it. Using mathematical equations he predicted the likelihood of finding electrons in certain positions. It is called the quantum Mechanical Model.

Sources:
http://www.abcte.org/files/previews/chemistry/s1_p6.html
https://www.biography.com/scientist/erwin-schrdinger

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In 1932, James Chadwick discovered the existence of the neutron at the core of the atom. He discovered this by bombarding beryllium with alpha particles from a polonium source. This radiation knocked loose protons from hydrogen atoms in that target and those protons recoiled with very high velocity. His discovery of a neutron leads to the existence of nuclear weapons.

Sources:
https://www.aps.org/publications/apsnews/200705/physicshistory.cfm
https://www.wired.com/2009/02/feb-27-1932-neutron-discovered-a-bomb-on-the-way-2/