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Democritus keep up with matter when small enough it reaches a point where it can no longer be divided and becomes Atomos(which mean indivisible). He also believed that atoms are different size, shape, were always moving colliding with each other in the void. Changes in matter were a result of dissociations or combinations of the atom. Democritus came up with his theory around 460 B.C. in Abdera, Greece.

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Dalton based his theory on the law of conservation of mass and law of constant composition. His theory is all atoms of a given element are identical in mass and properties, compounds are combinations of two or more different types of atoms, and a chemical reaction is a rearrangement of atoms. Dalton developed his atomic theory in 1808 in Manchester.

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Mendeleev created the periodic system to organise the elements and he put them in order by increasing atomic weight.He predicted the properties of more elements so he left space in the table for the unknown elements. The periodic table was created in 1869 in Russia.

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Thomson discovered that atoms have electrons and that electrons are smaller than atoms. Thomson's experiments with cathode ray tubes which showed that all atoms contain tiny negatively charged subatomic particles. This model was created in 1904 in Cambridge.

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Ernest Rutherford discovered that atoms had a nucleus and that is where all the positive charge in an atom was concentrated in. He also did a gold foil experiment that showed that an atom is mostly empty space with tiny positively charged nucleus. He discovered this in 1911 in Manchester.

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Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to lower orbit. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. This theory was discovered in 1913 in Copenhagen University.

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Erwin Schrodinger (Quantum Mechanical Model)

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Erwin Schrodinger (Quantum Mechanical Model)

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Schrodinger combined the equations for the behavior of waves with the de Broglie equation to generate a mathematical model for the distribution of electrons in an atom. The quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron but it portrayed as a nucleus surrounded by an electron cloud. He discovered this theory in 1920 in the University of Vienna.