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. Source of a particles Beam of Some a particles are scattered Most particles pass straight through foil Thin metal foil Screen to detect scattered a particles Alpha particle = two protons and two neutrons Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

Appling the Results to the Models Nuclear atom Nucleus + - Alpha particles Plum-pudding atom

Models of the Atom - Greek model (400 B.C.) Dalton’s model (1803) "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg e + + - Greek model (400 B.C.) Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) “Models of the Atom”   Description: This slide shows he evolution of the concept of the atom from John Dalton to the present. Basic Concepts ·         The model of the atom changed over time as more and more evidence about its structure became available. ·         A scientific model differs from a replica (physical model) because it represents a phenomenon that cannot be observed directly. Teaching Suggestions Use this slide as a review of the experiments that led up to the present-day view of the atom. Ask students to describe the characteristics of each atomic model and the discoveries that led to its modification. Make sure that students understand that the present-day model shows the most probable location of an electron at a single instant. Point out that most scientific models and theories go through an evolution similar to that of the atomic model. Modifications often must be made to account for new observations. Discuss why scientific models, such as the atomic models shown here, are useful in helping scientists interpret heir observations. Questions Describe the discovery that led scientists to question John Dalton’s model of the atom ad to favor J.J. Thomson’s model. What experimental findings are the basis for the 1909 model of the atom? What shortcomings in the atomic model of Ernest Rutherford led to the development of Niels Bohr’s model? A friend tells you that an electron travels around an atom’s nucleus in much the same way that a planet revolves around the sun. Is this a good model for the present-day view of the atom? Why or why not? Another friend tells you that the present-day view of an electron’s location in the atom can be likened to a well-used archery target. The target has many holes close to the bull’s-eye and fewer holes farther from the center. The probability that the next arrow will land at a certain distance from the center corresponds to the number of holes at that distance. Is this a good model for the present-day view of the atom? Why or why not? Suppose that, it the future, an apparatus were developed that could track and record the path of an electron in an atom without disturbing its movement. How might this affect the present-day model of the atom? Explain your answer. How does developing a model of an atom differ from making a model of an airplane? How are these two kinds of models the same? Drawing on what you know in various fields of science, write a general statement about the usefulness of scientific models. Bragg and his father, Sir W.H. Bragg, shared the 1915 Nobel prize in physics for studies of crystals with X-rays. Charge-cloud model (present) Bohr’s model (1913) Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Models of the Atom - Dalton’s model (1803) Greek model (400 B.C.) + + - Dalton’s model (1803) Greek model (400 B.C.) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) 1803 John Dalton pictures atoms as tiny, indestructible particles, with no internal structure. 1897 J.J. Thomson, a British scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a sphere of positive electric charge. 1911 New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus. 1926 Erwin Schrödinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model. 1913 In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus. “Models of the Atom”   Description: This slide shows he evolution of the concept of the atom from John Dalton to the present. Basic Concepts ·         The model of the atom changed over time as more and more evidence about its structure became available. ·         A scientific model differs from a replica (physical model) because it represents a phenomenon that cannot be observed directly. Teaching Suggestions Use this slide as a review of the experiments that led up to the present-day view of the atom. Ask students to describe the characteristics of each atomic model and the discoveries that led to its modification. Make sure that students understand that the present-day model shows the most probable location of an electron at a single instant. Point out that most scientific models and theories go through an evolution similar to that of the atomic model. Modifications often must be made to account for new observations. Discuss why scientific models, such as the atomic models shown here, are useful in helping scientists interpret heir observations. Questions Describe the discovery that led scientists to question John Dalton’s model of the atom ad to favor J.J. Thomson’s model. What experimental findings are the basis for the 1909 model of the atom? What shortcomings in the atomic model of Ernest Rutherford led to the development of Niels Bohr’s model? A friend tells you that an electron travels around an atom’s nucleus in much the same way that a planet revolves around the sun. Is this a good model for the present-day view of the atom? Why or why not? Another friend tells you that the present-day view of an electron’s location in the atom can be likened to a well-used archery target. The target has many holes close to the bull’s-eye and fewer holes farther from the center. The probability that the next arrow will land at a certain distance from the center corresponds to the number of holes at that distance. Is this a good model for the present-day view of the atom? Why or why not? Suppose that, it the future, an apparatus were developed that could track and record the path of an electron in an atom without disturbing its movement. How might this affect the present-day model of the atom? Explain your answer. How does developing a model of an atom differ from making a model of an airplane? How are these two kinds of models the same? Drawing on what you know in various fields of science, write a general statement about the usefulness of scientific models. Timeline: Wysession, Frank, Yancopoulos Physical Science Concepts in Action, Prentice Hall/Pearson, 2004 pg 114 1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1924 Frenchman Louis de Broglie proposes that moving particles like electrons have some properties of waves. Within a few years evidence is collected to support his idea. 1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons. 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

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.