The Discovery of the Atom

Democritus said that all material things are made of extremely small, irreducible particles, called 'atoms'. “Nothing exists except atoms and empty space; everything else is opinion” His atomic theory was rejected by almost everybody, including Arisotle, for the next two millenia.
http://library.thinkquest.org/13394/media/demokryt.jpg

J. J. Thomson modified Crooke's tube () by using a magnetic field, and particles would bend down toward the positive charges. Thompson then concluded that atoms must contain negatively charged particles; discovering the electron. He also made the Plum Pudding model of an atom where the electrons were evenly spread throughout a sphere of positvely charged material.
http://www.nobelprize.org/nobel_prizes/physics/laureates/1906/thomson.jpg

Ernest Rutherford showed that the atom was a lot of empty space, with a small positively charged nucleus that contained most of the mass, and a very small mass of negatively charged particles orbiting the nucleus. He also found that for the atom's mass to be what it is, there must be neutral particle with the same mass of the proton. "After the discovery of the electron, it was realized that there must be positive charge centers within the atom to balance the negative electrons and create electrically neutral atoms. Rutherford's discovery of the nucleus demonstrated that these positive charges were concentrated in a very small fraction of the atoms' volume."
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford.jpg

Niels Bohr created the first successful model of an atom. The model showed that the electrons orbit the nucleus just like the planets orbit the sun. This is incorrect however, considering that it is impossible to know exactly how they are actually moving.
http://www.chemicalelements.com/bohr/b0013.gif http://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Niels_Bohr.jpg/180px-Niels_Bohr.jpg

James Chadwick proved the existance of neutrons by bombarding beryllium with alpha particles. While doing this, he noticed a particle with a mass very close to that of a proton being released. Considering the fact that they were not bent by electrical fields and were highly penetrating, these particles were electrically neutral.

http://web.me.com/dtrapp/people2.f/Chadwick.jpg