624-574 BC

          Thales of Miletus

          States that water is the simplest substance on Earth


500-428 BC

          Anaxgoras and Empedocles

        Changes in matter are due to changes in indivisible particles


484-428 BC


        Separated indivisible matter into four elements; earth, water, fire and air.


460-370 BC


Matter is made of small indivisible particles which have different form, position and arrangement.  Democritus named these particles atoms.


384-322 BC


Gathered all theories up to date and was responsible for formalizing the knowledge into one idea.  Aristotle was leery that Democritus’s atoms didn’t account for the great variance of matter.



          Rodger Bacon

Taught that in order to understand the natural universe there must first be observation, allowing for evidence to come from the natural world.



          Isaac Newton

The universe is a mechanical universe with small solid masses in constant motion



          John Dalton

Developed an atomic theory stating that spherical atoms had measurable properties of mass



          Michael Faraday

Split molecules with electricity by means of electrolysis, developed laws of electrolysis



          J. Plucker

         Built the first cathode ray tube, used for gas discharge



          James Clerk Maxwell

        Magnetic and electric fields filled “empty” space in atoms



          Sir William Crookes

Studied the properties of cathode rays and found that they exhibit negative charge and mass.



          E. Goldstein

<>          Used cathode ray tubes to discover that “canal rays” have opposite properties of electron.


          Wilhelm Roentgen

Used cathode ray tubes to observe that nearby chemicals glowed and the penetrating rays coming from the cathode ray tube were not affected by magnetic fields.  He named these rays, X-Rays.



          Henri Becquerel

Discovered that some chemicals spontaneously decompose and give off penetrating rays while working with X-rays and photography paper.



          J.J. Thomson

Discovers the electron and used cathode ray tubes to determine the charge to mass ratio of an electron to be 1.759 x 108 Coulombs/gram.

          J.J. Thomson

        Found canal rays were associated with a proton, H+.



          Marie Skodowska Curie

Named the spontaneous decay process of uranium and thorium to be radioactivity.



          Ernest Rutherford

          Discovers alpha and beta rays emitting from radium.




Discovered that radioactive elements have isotopes, and half lives.  Made preliminary calculations concerning the energy released during decay.

          Max Planck

          Used quanta, discrete units of energy to explain glowing hot matter.

          Pierre Curie

          While working on radioactive substances, Curie discovers gamma rays.




Saturnian model of the atom where there is flat rings of electrons revolving around a positively charged particle.




          Inert gases have stable electron configurations.

          Ernest Rutherford

        Discovers alpha rays are heavily positively charged particles.



          Albert Einstein

Publishes theory on special relativity and states that matter can be converted into energy.



          Hans Geiger

          Used an electrical device to click when hit with an alpha particle.



          R.A. Millikan

Performed an oil drop experiment to determine the charge and the mass of an electron to be 1.602 x 10-19 C and 9.11 x 10-28 g respectively.



          Ernest Rutherford

Performed the gold foil experiment to determine that the nucleus is a small, dense, and positively charged part of the atom, based on the assumption that electrons are on the outside of the nucleus. Develops the plum pudding model of the atom.



          Neils Bohr

        Publishes paper regarding combining nuclear and quantum theories.



          H.G.J. Moseley

Determined the charge of the nuclei of most atoms using X-ray tubes.  Stated that the atomic number is equal to the number of protons in the nucleus.




        Used mass spectra to prove the existence of isotopes.

          Ernest Rutherford

First artificially induced nuclear reaction.  Used nitrogen and alpha particles to obtain an oxygen isotope and protons.



          Neils Bohr

Created a model of successive orbital shells based o an equation of fixed radii and quantum numbers.



          Louis Victor de Broglie

        Electrons are similar to both particles and waves.



          Werner Heisenberg, Max Born and Erwin Schodinger

        Develop quantum mechanics



          Werner Heisenberg

Used the frequencies based on spectra lines to describe atoms.  The Principle of Indeterminacy states that you can not know the both the position and the velocity of a particular particle at any one moment in time.



          Erwin Schrodinger

Saw electrons as a continuous cloud and developed wave mechanics to be a mathematical model for the atom.

          Paul Dirac

Suggested the existence of anti-particles.  Found the first anti-electron, positron.



          James Chadwick

Discovered neutral atomic particle with similar mass to a proton, the neutron by using alpha particles.

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