History of the Atom timeline.

Democritus made a theory. This theory stated that all matter consists of invisible particles called atoms. It also stated that atoms are indestructible and they are solid but invisible. Atoms are all the same, but they can differ in size, shape, mass, position, and arrangement. He told us that solids are made of small, pointy atoms and liquids are made of large, round atoms. Finally, he told us that oils are made of very fine, small atoms that can easily slip past each other.

John Dalton was an English chemist that created the Atomic Theory of Matter. He got this idea by Democritus and his findings. He included in this theory that all matter is made of atoms and that atoms cannot be created or destroyed. He also said that atoms of different elements combine in whole ratios to form chemical compunds. His theory later contributed to an advance in the atomic model.

J.J. Thomson was a very important scientist when it came to the atomic model. Before him, all models of the atom looked like a big solild ball. J.J. Thomson discovered the electron, which led him to create the "plum pudding" atomic model. In this model, he thought that the atom was mostly positive, and negative electrons wandered around the atom.

Marie and Pierre Curie were a European couple that contributed to atomic chemistry by exploring the unknown facts about radioactivity. After radiation was discovered, Marie decided to look further into this discovery. Through this she and her husband discovered the elements radium and polonium and won the two Nobel Peace Prize's for their works in radioactivity.

Ernest Rutherford was another scientist that changed the atomic model. He felt that J.J. Thomson's model was incorrect, so he created a new one. He created the nucleus, and said that instead of the positive matter being the whole atom, it was just in the middle. He said the atom was mostly empty space and that the electrons surrounded the positive nucleus. This model influenced one of his own students to perfect the atomic model later on.

Niels Bohr was a Danish scientist that was a student of Rutherford. He decided to make a new model based off of Rutherford's model, but changed the orbit of the electron. Also, he created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom. Bohr also used Planck's ideas in order to create quantum mechanics, his new concept regarding energy. This model is still used to this day.

Jame Chadwick was an English scientist that discovered the neutron. Before this discovery, Rutherford had concluded that the nucleus was made of positive matter. It made sense that the atom was neutral because the negative electrons and the positive protons cancelled out. But, Chadwick started to question why there was a difference between the atomic mass and the number of protons. Chadwick then found that the missing component was a neutral part: the neutron.