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He started contributing to the atom around 400 B.C.E when he first created the idea that the atom is the building block of matter. The image shown above is the model that Democritus created to show what he thought an atom looked like. He believed that atoms were only round spheres. His ideas of the atom were that they are minuscule, solid particles that can't be destroyed, that all matter is made up of them, and that all atoms separated by a void. He also thought that atoms are constantly moving, and different types of matter had different types or arrangements of atoms.

Fun Fact: He was known as the "laughing philosopher" due to how joyful he looked while working.

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John Dalton contributed to the atom in the year 1803 to 1827, and came up with the atomic theory and also wrote a book titled "New System of Chemical Philosophy" that talked about compounds. Above is the model he created to represent the atom, and he assumed that they looked like billiard balls. Dalton believed that all matter is made up of indivisible and indestructible atoms, the atoms of an element are all identical, and different elements had atoms with different mass and different chemical properties. He also believed that the atoms of different elements are able to combine in whole numbers to create compounds, and atoms can not be created or destroyed, so whenever a compound decomposed, its atoms didn't change.

Fun Fact: Dalton had also studied color blindness, which is sometimes referred to as Daltonism to honor him.

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J.J. Thomson started contributing to the atom in 1897 when he discovered the electron, while experimenting with a cathode ray tube and found out they were negatively charged.  In 1904, he created the model presented above, which shows electrons positioned by electrostatic forces, in a sphere made up of positive energy. He referred to the model as the "plum pudding" model. He believed that atoms were made up of positive matter that contained electrons, which are negatively charged particles, scattered throughout. He also used a mass spectrograph to view what he believed to be isotopes, which were atoms of the same element, but with different atomic weights.

Fun Fact: J.J. Thomson won the Nobel Prize in Physics in the year 1906.

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He first started contributing to the atom in 1909, where he conducted an experiment with gold foil and alpha particles, and noticed while majority of the particles went through the gold, some would bounce back. The model above is what he created as an interpretation of the atom, and it is to show the electrons located outside of the positively-charged nucleus. From this experiment, he believed that there was a positively-charged nucleus in the atom along with electrons that are located outside the nucleus. He also conducted another experiment with nitrogen and alpha particles by bombarding the nitrogen with the alpha particles, causing them to create a new element, oxygen. He believed that atoms are not indestructible after his experiment. It was believed at the time that he split an atom.

Fun Fact: He had collaborated with J.J. Thomson to study the ionization of gases created by X-Rays.

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Niels Bohr started contributing to the atom in 1913 where he created the model above. He created this model because he believed that the electrons are able to have stable orbits around the nucleus. He also believed that the energy of an electron depended on the size of the orbit in the atom, and electrons were slower for smaller orbits. Bohr had proposed that these electrons were confined to these orbits, and an electron could suddenly jump between orbits by absorbing or releasing a photon with the appropriate wavelength; if electrons were to jump to a lower-energy orbit, radiation would be sent out is another theory he believed. He made these theories based off the hydrogen atom.

Fun Fact: When he was studying in University, he had caused explosions in the lab while doing experiments.

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Erwin Schrodinger contributed to the atom in 1926 when he created the quantum mechanical model. He created this model to predict where an electron could be located, rather than pin down its exact location. Schrodinger believed that electrons can't be found in a specific location, but predictions can be made on where the electron is. He also believed that where the cloud of the atom was most dense, there was a higher probability of finding the electron, and when the cloud was less dense, there was a lower chance. This introduced the idea of sub-energy levels.

Fun Fact: He created the paradox known as "Schrodinger's cat."