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The Experiment • To test this he designed and experiment directing ‘alpha’ particles toward a thin metal foil. – The foil was coated with a substance that produced flashes when it was hit by an alpha particle. Some a particles are scattered Source of a particles Beam of a particles Screen to detect scattered a particles Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56 Most particles pass straight through foil Thin metal foil Appling the Results to the Models Alpha particles Nucleus + + - - + + - + + - + - + - - Plum-pudding atom Nuclear atom Models of the Atom "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg e e + e + e + + e +e + e e + e + e Dalton’s Greek model model (400 (1803) B.C.) Thomson’s plum-pudding model (1897) Bohr’s model (1913) Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125 - - + Rutherford’s model (1909) Charge-cloud model (present) Models of the Atom e e + e - + e + + e +e +e e + e + e Dalton’s model Greek model (1803) (400 B.C.) 1803 John Dalton pictures atoms as tiny, indestructible particles, with no internal structure. 1800 - Thomson’s plum-pudding model (1897) - + Rutherford’s model (1909) 1897 J.J. Thomson, a British 1911 New Zealander scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a sphere of positive electric charge. Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus. 1805 ..................... 1895 1900 1905 1910 1904 Hantaro Nagaoka, a Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around Saturn. Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125 1915 Bohr’s model (1913) 1926 Erwin Schrödinger 1913 In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus. 1920 1925 Charge-cloud model (present) 1930 develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model. 1935 1940 1945 1924 Frenchman Louis 1932 James de Broglie proposes that moving particles like electrons have some properties of waves. Within a few years evidence is collected to support his idea. Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons. Bohr Model Neils Bohr Planetary model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus. • Bohr’s contributions to the understanding of atomic structure: 1. Electrons can occupy only certain regions of space, called orbits. 2. Orbits closer to the nucleus are more stable — they are at lower energy levels. 3. Electrons can move from one orbit to another by absorbing or emitting energy, giving rise to characteristic spectra. • Bohr’s model could not explain the spectra of atoms heavier than hydrogen. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.