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History of the Atom Objective: know the historical development of the atom by reading and presenting facts. In groups of 4 or 5, you will read a portion of a test book or article about a scientist who made a major contribution to the history of the atom. We will do the first one together as a class. Who: Democritus When: 2500 years ago or 488 B.C.E What was their contribution: Defined the word "atomos" or tiny indivisible, solid objects that make up all matter in the universe. Our current word of atoms comes from his definition If possible, how did they determine their contribution: he used logic and never performed an experiment, as a result the theory was rejected until 2000 years later Additional Information deemed important: believed the atomos were of different varieties of different shapes, sizes and masses (elements) Who: John Dalton When: Late 1700s to early 1800s (1803) What was their contribution: Developed the modern atomic theory and the concepts of atomic weight If possible, how did they determine their contribution: Experimentation and proposing the law of multiple proportions (when two elements react to form more than one substance the ratio of the masses is in small whole numbers, 1:1, 2:1) Additional Information deemed important: Dalton's Theory 1. matter is made of tiny particles - atoms 2. atoms are indivisible; during reactions atoms are rearranged, they do not break apart and are not created nor destroyed 3. all atoms of a given element are identical in mass and other properties (no longer true) 4. atoms of different elements differ in mass and other properties 5. atoms of one element can combine with atoms of another element to form compounds Who: J.J Thomson When: 1897 What was their contribution: discovered the proton and electron created the plum-pudding model of the atom If possible, how did they determine their contribution: The use of a cathode ray; the negative particles were attracted to the positive portion Additional Information deemed important: His plum-pudding model was disproved by Rutherford years later Who: Ernest Rutherford When: 1912 What was their contribution: used gold-foil experiment to come up with the positively charged nucleus and disproved the plum-pudding model If possible, how did they determine their contribution: he used the goldfoil experiment - negative particles fired at gold-foil, thought they would pass through but several particles bounced back (indicating a positive center to an atom) Additional Information deemed important: He was able to detect positive and negative portions of an atom, but was unable to find the neutrons (that came later) Who: Henry Moseley When: 1912 What was their contribution: theorized that the elements on the periodic table should be arranged by atomic number (protons) instead of atomic mass If possible, how did they determine their contribution: used x-ray spectra (wavelengths to determine the atomic number Additional Information deemed important: he was able to adjust the previous periodic table Who: Niels Bohr When: 1913 What was their contribution: modern atomic model (Bohrs Model) If possible, how did they determine their contribution: through quantum theory of atoms or there are energy level in atoms and atoms are in a stationary or stable state; to change states, energy must be added or released Additional Information deemed important:became a nuclear physicists, helped develop the idea of fission and the atomic bomb; became an advocate for the peaceful use of nuclear energy Who: James Chadwick When: 1932 What was their contribution: discovery of the neutron If possible, how did they determine their contribution: when working with beryllium and using the conservation of energy and momentum he was able to detect the neutron Additional Information deemed important: was a major contributor in the assembly of the atomic bomb History of the Atom Timeline