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The Atomic ModelHistory of the AtomKHALIL SINCLAIR500 BC AlchemistsEgypt/ChinaDeveloped the theory that all metals are composed of mercury and sulfur. Its possible to change base metals into gold. In Egypt it was a metallurgical craft.460 BC DemocritusGreeceInferred that everything was made up of atoms. Atoms were physically indivisible; between atom lies empty space. Atoms are indestructible and they are always moving. Infinite amount of atoms, different shapes and sizes3380 BC Aristotle GreeceBelieved that all materials on Earth were created by four elements, which include earth, water, air, and fire. He believed that all substances were made up of small amounts of these four elements of matter.1787 1789LavoisierFranceIn Methods of Chemical Nomenclature, he invented the system of chemical nomenclature which is still largely used today; sulfuric acid, sulfates, and sulfites.Elementary Treatsie of Chemistry was the first modern chemical textbook. It presented a unified view of new theories of chemistry as it contained the Law of Conservation of Mass.1803 DaltonEnglish chemist who proposed the Atomic Theory which stated that all matter was composed of small indivisible particles called atoms. These atoms possess unique characteristics and weight for elements. He also stated that three types of atoms exist: simple, compound, and complex.

Daltons atomic model. A round hollow sphere with neither electrons, protons, or neutrons.1821 1832FaradayEnglandPublished his work on electromagnetic rotation; principal behind the electric motor.Discovered electromagnetic induction. This is the principle behind the electric transformer and generator.1869 MendeleevSiberiaPublished the periodic table in the Principles of Chemistry. The elements were arranged by their atomic mass.1869 Mendeleev

Mendeleevs version of the periodic table1873 James Clark MaxwellEnglandCreated a set of four linear partial differential equations that summarize the properties of the electromagnetic field.

1879 CrookesEnglandDeveloped the Crookes tube and in it produced cathode rays for the first time. He also invented the radiometer and spinthariscope.

1887 HertzGermanyDiscovers the photoelectric effect. Hertz also demonstrates experimentally that electromagnetic waves exist.1895 Wilhelm RoentgenGermanyDiscovers X-rays in experiments which electron beams in plasma

One of the first X-rays1896 Henri BecquerelFranceFound that rays coming from uranium ore affected a photographic plate like X-rays did. The rays were not from an external source and were more powerful than those from a sample of pure uranium. His experiment led to the discovery of natural radioactivity.1897 ThomsonEnglandFirst to measure the electron. Developed the plum pudding model of the atom. Describes the atom as a slightly positive sphere with small electrons inside.

1898 RutherfordNew Zealand/EnglandInferred a small, dense, positively-changed nucleus as the result of the alpha particle scattering gold foil experiment.1898 CurieFranceStudied and identified radioactive elements. Discovers that rays emitted by uranium salt samples make surrounding air electrically conductive, and measures the emitted rays intensity. Through a systematic search of substances. Curie finds that thorium compounds, like those of uranium, emitted Becquerel rays.1900 Max PlanckGermanyStated that radiation is quantized. Suggests that electromagnetic energy could only be emitted in quantized form, the energy could only be a multiple of an elementary unit E = hv, where h is Plancks constant and v is the frequency of the radiation.1905 EinsteinSwitzerlandProposed the quantum of light in which he states that light behaves like a particle but also has wave nature, giving it a dual nature. Stated the equivalence of energy and mass through the theory of special relativity.

1908 GeigerGermanyDemonstrated that energy and mass are conserved in atomic processes1909 MillikanUnited StatesPublishes the results of his oil drop experiment, in which he precisely determines the electric charge of the electron. Determination of the fundamental unit of electric charge makes it possible to calculate the Avogadro constant (the number of atoms or molecules in one mole of any substance) and thereby to determine the atomic wright of the atoms of each element.

1911 RutherfordNew Zealand/EnglandBohr joined Rutherford. He found that the ratio of energy in electrons and the frequency of their orbits around the nucleus was equal to Plancks constant. It was suggested the revolutionary idea that electrons jump between energy levels in quantum fashion, that is, without ever existing in and in-between state. When an atom absorbs or gives of energy the electron jumps to higher or lower orbits.

1914 MoseleyTurkeyStudying under Rutherford, Moseley developed the application of X-ray spectra to study atomic structure. He measured the wavelengths of the X-rays given off by certain metal. Moseley was able to determine the number of positive charges in the nucleus of an atom. This was the first concept of the atomic number.1922 BohrDenmarkStated that electrons could only orbit the nucleus is successively larger orbits around the nucleus. The outer orbits could hold more electrons. The electrons in the outermost shell determine the properties of the atom. He also inferred that when an electron moves from an outer orbit to an orbit it emits a quantum of energy, a photon, in the form of light.1923 de BroglieFrancePublished the theory of particle-wave duality. This explains how electrons possessed a dual nature; properties of both particles and waves1923 MillikanUnited StatesFurther developed the atomic model with his work on the elementary charge of electricity and on the photoelectric effect. This was the possible ejection of the electrons from the surface, usually a metal, in response to incident light.1925 Wolfgang PauliGermanyOutlines the Pauli exclusion principle which states that no two identical fermions may occupy the same quantum state simultaneously, a fact that explains many features of the periodic table.1927 HeisenbergDenmarkHe determined that the only way to describe the location of an electron in an atom is through probability distribution. This principle of his forms the basis of the electron cloud model.1930 SchroedingerAustraliaDeveloped wave mechanics, which describe the behavior of quantum systems for bosons. Through the creating of his model, he was able to further increase ones knowledge of the atom by providing more information on its atomic structure (i.e., the electron cloud)1932 ChadwickEnglandDiscovered the neutron using evidence collected by Irene Curie, who discovered that when beryllium was bombarded, with positively charged alpha particles a beam with a high penetrating power was created. Chadwick discovered that this beam was not deflected by either electric or magnetic fields, meaning it contained neutral-particles; neutrons.1938 Meitner, Hahn, StrassmanGermanyFound that when Uranium is bombarded by neutrons it produces smaller nuclei roughly half the size of the original Uranium nucleus. Realized that Hahn was splitting the Uranium nucleus, something never done before.

Bibliographyhttp://atomictimeline.net/index.phphttp://www.softschools.com/timelines/atomic_theory_timeline/95/http://hi.fi.tripod.com/timeline/http://en.wikipedia.org/wiki/Atomhttp://www.barcodesinc.com/articles/timeline-on-atomic-structure.htm