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Lived Around 450 BC

Democritus theorized that all thing are made up of atoms.

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In 1898
Marie Curie announced the discoveries of two new elements radium and polonium. She also discovered that radioactivity depended on the amount of element present in the compound.

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Created the Rutherford atomic model (a description of atoms) in 1911. The model described the atom's nucleus, and that the atom is mostly comprised of empty space.

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In 1913,
Bohr theorized that electrons move around the proton in specific orbits. Also, he thought that whenever an electron moved to a lower energy level, light was emitted.

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Around the year 1803

Dalton added on to Democritus's theory by saying that atoms are indestructible and indivisible, all atoms of the same element are identical, and  that compounds are formed by a combination of two or more different kinds of atoms.

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In 1897, Thomson came to the conclusion that all matter contains masses contains particles of the same kind but are smaller than atom in which they form a part. This is known as the electrons.

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Theorized in the 1930s that atoms are not only made up of protons, but also neutrons.