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Democritus expressed his belief that matter consists of small, solid particles which he called atoms. He discovered that these small objects cannot be broken down any further. Democritus concludes that all things are composed of atomos that are too small to be seen by the unaided eye. He explains that there are empty spaces between the inner structures of matter which have a uniform structure all throughout yet still vary in shape, size, and weight.

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In the early 1800s, the English Chemist, John Dalton, conducted a number of experiments that eventually led to the acceptance of the idea of atoms. He concluded that atoms are invisible and indestructible particles. He deduced that all elements are composed of atoms, therefore, atoms of the same elements are exactly alike meanwhile atoms of different elements are different. On the other hand, compounds are formed by the joining of atoms of two or more elements.

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Dalton's Atomic Model is the first representation that matter is made up of atoms which he represented with solid, hard spheres.

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In 1897, the English scientist, Joseph John Thomson, provided the first hint that atoms are made up of even smaller particles.
The Cathode Ray Experiment ─ J.J. Thomson applied a high electric voltage on both ends of the tubs between two electrodes. He observed a particle stream coming out of the negatively charged electrode the positively charged electrode.

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Joseph John Thomson's Atomic Model (Plum Pudding Model)

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Joseph John Thomson's Atomic Model (Plum Pudding Model)

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J.J. Thomson's Atomic Model represents an atom being a positively-charged structure with negatively-charged electrons scattered like plum in a pudding.

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In 1908, the English Physicist, Ernest Rutherford, performed his gold foil experiment and discovered another subatomic particle.
The Gold Foil Experiment ─ This experiment involved firing a stream of positively-charged particles at a thin sheet of gold foil. He observed that most of the alpha particles passed through with little or no deflection, meanwhile a few either deflected at large angles or bounced back.

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Ernest Rutherford's Atomic Model (Nuclear Model)

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Ernest Rutherford's Atomic Model (Nuclear Model)

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In the nuclear atom, the protons and neutrons that comprise nearly all of the mass of the atom, are located in the nucleus at the center. The electrons are located outside of the nucleus and occupy most of the volume of the atom.

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In 1913, the Danish scientist, Niels Bohr, proposed an improvement.

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Niels Bohr's Atomic Model (The Planetary Model)

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Niels Bohr's Atomic Model (The Planetary Model)

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In his model, he placed each electron in a specific energy level. Electrons move in definite orbits around the nucleus much like planets circle in the sun. The orbits, or energy levels, are located at certain distances from the nucleus.

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In 1926, the Austrian physicist, Erwin Schrödinger, took the Bohr atom model one step further. He used mathematical equations to describe the likelihood of finding an electron in a certain position.

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Erwin Schrodinger's Atomic Model (The Quantum Mechanical Model)

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Erwin Schrodinger's Atomic Model (The Quantum Mechanical Model)

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Unlike the Bohr model, this does not define the exact path of an electron but rather predicts the odds of the location of the electron.

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In 1932, an unknown radiation was produced. James Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton.