Dustin George

Democritus:
A Greek philosopher who theorized that all matter consisted of tiny particles that he called atoms, a name derived from two Greek words meaning indivisible. He proposed the earliest views on the shapes and connectivity of atoms. He reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials. Democritus was the main proponent of this view. Using analogies from our sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts. Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets.
His contributions broke open the world of the atom.

Joseph Proust:
A French chemist and apothecary who came up with the Law of Definite Proportions which states that a chemical compound always contains exactly the same proportion of elements by mass. Proust studied copper carbonate, the two tin oxides,and the two iron sulfides to prove this law. He did this by making artificial copper carbonate and comparing it to natural copper carbonate. With this he showed that each had the same proportion of weights between the three elements involved (Cu, C, O). Between the two types of the other compounds, Proust showed that no intermediate indeterminate compounds exist between them.
His law was adapted as part of Dalton's Theory.

John Dalton:
An English chemist, meteorologist, and physicist who theorized that:
1.Elements are made of tiny particles called atoms.
2.The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.
3.All atoms of a given element are identical.
4.Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms.
5.Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.
He reached this theory after a series of experiments that included the analysis of a number of pure compounds and the study of gases.
John Dalton contributed an early understanding of the behavior of atoms that has had to be refined a little bit sense his stating it.

J.J. Thomson:
A British physicist who discovered the electron in 1897, the first subatomic particle discovered. Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field . By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was able to measure the particle's mass. This showed that cathode rays were matter, but he found that the particles were about 2000 times lighter than the mass of the lightest atom, hydrogen. He concluded that the rays were composed of very light negatively charged particles which he called "corpuscles".
His experiments were the first to introduce a subatomic particle, opening the way for more.

Ernest Rutherford :
A New Zealand-British chemist and physicist who discovered Protons. He discovered this through an experiment where a beam of alpha particles, generated by the radioactive decay of radium, was directed normally onto a sheet of very thin gold foil. The gold foil was surrounded by a circular sheet of zinc sulfide (ZnS) which was used as a detector: the ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. Through this experiment he was able to say that there had to be a nucleus where a large amount of the atom's charge and mass is concentrated into a very physically-small (as compared with the size of the atom) region. He also theorized about the existence of neutrons.
His discovery of the proton opened the way for a neutrally charged particle to balance it.

James Chadwick:
An Englishman who discovered the neutron. His experiment was based on the 1930 discovery that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation. However, further investigations into the properties of the radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and since they were not deflected upon passing through a magnetic field, neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect). Irene Curie and her husband discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin, protons were knocked loose which could be easily detected by a Geiger counter.
In 1932, Chadwick proposed that this particle was Rutherford's neutron. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton.
His discovery completed the knowledge of the main subatomic particles in the atom today.

Niels Bohr:
A Danish physicist who made fundamental contributions to understanding atomic structure, he is most credited with the Bohr Atomic Model. He published this model introducing the theory of electrons traveling in orbits around the atom's nucleus, with the chemical properties of the element being largely determined by the number of electrons in the outer orbits. He reached this model by adapting Rutherford's nuclear structure to Max Planck's quantum theory.
His model of the atom is still used as a basic model today and started the process toward the Quantum Model.

Carl David Anderson :
An American physicist who discovered the positron. He began investigations into cosmic rays during the course of which he encountered unexpected particle tracks in his cloud chamber photographs that he correctly interpreted as having been created by a particle with the same mass as the electron, but with opposite electrical charge. This discovery, announced in 1932 and later confirmed by others, validated Paul Dirac's theoretical prediction of the existence of the positron. Anderson obtained the first direct proof that positrons existed by shooting gamma rays produced by the natural radioactive nuclide ThC'' (208Tl) into other materials, resulting in creation of positron-electron pairs.
His contributions proved the existance of antimatter and adds to the understanding of how atoms behave.