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Democritus believed that the atoms were specific to each type of material that they composed. He also believed that the atoms were solid, hard, indestructible, were in constant motion, and it had different shape for each type of material.

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Aristotle didn't believed in the atomic model that Democritus had proposed before. He believed in a completely different model based in the four nature elements. He thought that all the substances and all the materials were made by small amounts of fire, water, air and earth.

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Between 1772 and 1794 years, Antoine Lavoisier discovered that during a chemistry reaction the masses of the reactants are conserved. With this theory he crated the Law of Mass Conservation fundamental for stoichiometry calculations. Not happy with his own law of conservation of matter, he discovered 11 more elements (chlorine, cobalt, hydrogen, manganese, molybdenum, nickel, nitrogen, oxygen, phosphorus, platinum, and tungsten).

His law of conservation of mass was the first scientific theory that chemistry had. He was the creator of the first table of elements and gave chemicals their modern names. Light and heat were still included in this table.

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John Dalton´s theory was that the elements were composed of indivisible solid balls named as atoms. All these atoms of the same element are identical to each other and atoms cannot be destroyed or created. This atomic model is known as ¨billiard ball model¨. Later he created symbols to this atoms, being the first scientist to do it.

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Joseph Proust proposed the law of definite proportions, which is that when two or more elements are put together to create a compound, they always do so in a constant mass ratio.

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He created a table that grouped elements into columns which he called "Families" and put elements with similar properties together.

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Eugene Goldstein model is Known as the Raising Pudding Model. In 1886 he discovered that the atoms have positive charge. Using a tube filled with hydrogen gas he observed particles moving in the direction of the cathode, a negative terminal. With that he concluded the presence of positive charge in the atoms.

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Ernest Rutherford proposed that the atom has a nucleus, in which the positive charges and the majority of the mass in the atom are concentrated. This positive charge is balanced by the negative charge of the electrons which are outside the nucleus.

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Millikan considered that all electric charge is due to the excess or deficiency of electrons. He also managed to measure the value of the charge of an electron using a method called ¨falling-drop¨, and at the same time its mass.

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Moseley demonstrated that the elements in the periodic table had to be ordered depending on the atomic number, because it is the fundamental feature that describes an atom. He demonstrated this with the use of Medeleev's law.

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He proposed that an electron has to revolve around a proton, which represented the nucleus of an atom. So the nucleus, composed by positive charge, is surrounded by the electrons, such as an solar system.

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Werner Heisenberg claimed that the exact position of an electron within an atomic nucleus at any given time could not be known with certainty, but could only be calculated statistically within a probability, which is known as the ¨uncertainty principle¨.

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James Chadwick demonstrated the existence of neutrons in atoms, which completed the atomic structure proposed by Rutherford.

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In 1904, J.J. Thomson proposed one of the most important atomic model known as the ¨Plum Pudding model¨. He proposed this model soon after the discover of the electron, but before the discover of the nucleus. His model is basic a cloud made by protons (positive charges) and electrons (negative charge). He assumed that the electrons were so much smaller than the proton, hence the name of this model (Plum Pudding).

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Erwin Schrödinger is known as the father os the Quantum Mechanics. In January of 1926, he published his theory about the ¨Waves Mechanics¨. With this equations, it is possible to calculate he 3D region of probability to find the electron.