Democritus was a Greek philosopher, who decided to ask, "What happens if you decide to break a piece of matter in half continually, until you can break it no further?" He thought there must be an end point somewhere. A smallest possible bit of matter. He called these particles "Atoms".
John Dalton was an English chemist born in 1766. Dalton is best known for his early work in the development of modern atomc thoery. John did much research on gases and the relationship between temperature and pressure. Through his many experiments he was able to come up with an early atomic theory which perisited of 5 "rules."
1. Elements are made of extremely small particles called atoms
2. Atoms of the same element are indentical in size, mass, and other properties. Atoms of different elements differ in these properties
3. Atoms cannot be subdivided created or destroyed.
4. Atoms of different elements combine in simple whole number ratios in order to form chemicla compunds.
5. In chemical reactions atoms are combined, seperated, or rearranged.
Sir Joseph John Thomson was an acclaimed British physicist from the late 1800"s to the mid 1900's. Thomson was credited for discovering the electron and isotopes within atoms, and was awarded the Nobel prize in 1906 for his discoveries. Thomson used a cathode ray to prove the existence of electrons. His experiment proved that the rays emitted were actually 1000 times smaller than what they thought was the smallest particle at the time, the hydrogen atom. This proved the theory of subatomic particles, and secured Thomson as a major contributor to the atomic model.
Max Karl Ernst Planck was a german theoretical physicist born in the mid 1800's. Planck was recognized for pioneering quantun theory when he recieved the Nobel prize in physics in 1918. Planck deveolped the theory that the frequency of a wave was related to is energy. He based this theory on the idea that the energy emitted from a resonator could only take on discrete values. Through his experiments he was able to derive the constant "h" we know today as Plancks constant.
Ernest Rutherford is most noted for his famous gold foil experiment. In his experiment, he used radium to produce alpha rays, which were projected on to a piece of gold foil. Behind the foil, there was a detection screen where he wouldbe able to see the alpha particles impact.
This determined that there must be some form positively charged matter must scatter the particles. He also inferred that atoms were mostly empty space, and that electrons orbited the positive center, much like planets around the sun.
Niels Bohr was a physicist born in Denmark in 1885. His model of the atom is the one that we currently use, and that best explains how atoms behave. He proposed that electrons can orbit at certain distances form the nucleus (Energy Levels) and that atoms can radiate energy when an electron changes energy levels.
Robert Andrews Millikan was born in 1868 in Morrison, Illinois. Millikan won the nobel prize in physics in 1928 for his work in finding the charge of an electron. Millikan was able to discover the charge in an experiment famously known as the "oil drop experiment." In this experiment Millikan suspended a charged oil drop between two metal electrodes, knowing the strength of the electric field he was able to determine the charge on the oil drop. Since J.J. Thomson has earlier discovered the charge to mass ratio of an electron it was then simple for Millikan to also calculate the mass of an electron.
Wolfgang Pauli was a Austrian theoretical physicist in the early 1900’s. Wolfgang made many advancements in quantum theory, he as praised by Einstein who landed him the nobel prize 1945. Wolfgang developed the exclusion principle, and the theory of nonrelativistic spin.
Louie deBroglie, a French physicist, is credited with the discovery of the "Wave-Particle Duality". This principle explains why light can travel as a wave, or a particele, but also, why electrons can travel like waves.
That picture represents the wave nature of electrons.
Erwin Schrodinger was an Austrian physicist born in the late 1800's. Schrodinger made many advancements in quantum mechanics, and developed the theory of wave mechanics. Schrodinger published a paper that gave the first wave equation for time independant systems. His famous "schrodingers cat" theory helped him to disprove the old Copenhagen theory.
Werner Heisenberg was a German physicist and mathematician who was born on December 5th, 1901. His most notable discovery was the development of the matrix mechanics formulation of quantum mechanics. He worked closely with Neils Bohr, who he had first met a lecture being given by Bohr. Heisenberg's matrixes could give out either the position or momentum of a particle, but never both. Heisenberg was responsible for probability maps.
James Chadwick was credited with the discovery of the neutron. He was an english physicist born in 1981. Using gamma rays, he was able to determine that uncharged particles with the approximate mass of a proton. This particle was dubbed the neutron. It explained the discrepancies between the the atomic number of an atom, and its atomic mass.