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ATOM, Dalton's Billiard Ball Model, Democritus' Atom,…
ATOM
STRUCTURE OF ATOM 
ELECTRON - An electron is a subatomic particle that is negatively charged. It can be either free (not linked to any atom) or tied to an atom's nucleus. Electrons in atoms reside in spherical shells of varying radii, which indicate different energy levels. The more the energy contained in the electron, the bigger the spherical shell.
PROTON - A proton is one of the three major particles that comprise an atom. Protons are present in the atom's nucleus. This is a small, dense area in the atom's core. Protons have a positive electrical charge of one (+1) and a mass of one atomic mass unit (amu), or approximately 1.67 10-27 kg.
NEUTRON - an uncharged fundamental particle with a mass almost equivalent to the proton and found in all known atomic nuclei except the hydrogen nucleus.
Ernest Rutherford's Atomic Theory (1910's) - Rutherford proposed a Gold Foil Experiment as he was not convinced of Thomson's atomic model. He then accidentally discovered a nucleus due to the positively charged alpha particles that failed to pass through the gold foil.
John Dalton's Atomic Theory (1800's) - Dalton adapted Democritus' atomic theory and developed a new atomic theory with his experiments and supported observations that eventually led to the acceptance of the idea of atoms.
Neil Bohr's Atomic Theory (1910's) - Bohr improved Rutherford's model and developed a new one similar to the solar system and discovered that electrons move in definite orbits around the nucleus.
JJ Thomson's Atomic Theory (1890's) - Thomson discovered electrons through his experiment with cathode-ray tube that led to the idea that atoms are not the smallest, but rather, we're made up of much smaller things—positively charged matter and negatively charged electrons existing inside of it.
Aristotle (300 BC) - Theorized that all matter was made of the element fire, air, water, and earth; and have a property of hot, cold, dry, and wet.
Democritus' Atomic Theory (400 BC) - The first philosopher to come up with the idea of the atom. He postulated that all matter was composed of tiny, indivisible, and indestructible units wherein he called these units atoms a Greek word for indivisible.
Erwin Schrödinger’s Atomic Theory (1920's) - Schrödiger's Wave Theory is the latest atomic theory wherein it was found out that electrons do not orbit in the planet-like orbits but rather in a so-called electron cloud around the nucleus.
James Chadwick's Atomic Theory (1930's) - Chadwick discovered that atoms consisted not only of protons and electrons but also neutrons—neutrally charged subatomic particles with the same mass as protons that occupy the nucleus of the atom.
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Democritus' Atom
a round sphere with no electrons, protons, and neutrons;
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Atoms of different substances differ in size, weight, and shape.
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