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Models of the Atom Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present)

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Presentation on theme: "Models of the Atom Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present)"— Presentation transcript:

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2 Models of the Atom Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) Greek model (400 B.C.) John Dalton pictures atoms as tiny, indestructible particles, with no internal structure 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 Hantaro Nagaoka, a Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around Saturn New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus 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 Erwin Schrödinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons e e e e ee e e e e

3 Dalton Model of the Atom Late 1700’s - John Dalton- England Teacher- summarized results of his experiments and those of others Combined ideas of elements with that of atoms in Dalton’s Atomic TheoryDalton’s Atomic Theory

4 Foundations of Atomic Theory Law of Definite Proportions The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. Law of Multiple Proportions If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first elements is always a ratio of small whole numbers. Law of Conservation of Mass Mass is neither destroyed nor created during ordinary chemical reactions.

5 Conservation of Atoms John Dalton Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page H 2 + O 2 2 H 2 O 4 atoms hydrogen 2 atoms oxygen 4 atoms hydrogen 2 atoms oxygen H H O O O O H H H H H H H2H2 H2H2 O2O2 H 2 O H2OH2O +

6 Legos are Similar to Atoms Lego's can be taken apart and built into many different things. H H O O O O H H H H H H H2H2 H2H2 O2O2 H 2 O H2OH2O + Atoms can be rearranged into different substances.

7 45 g H 2 O? g H 2 O Conservation of Mass Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 204 High voltage Before reaction electrodes glass chamber 5.0 g H 2 80 g O g (mass of chamber) g total H2H2 O2O2 High voltage After reaction 0 g H 2 40 g O g (mass of chamber) g total O2O2 H2OH2O

8 Law of Definite Proportions g of lead 1.56 g of sulfur g of lead sulfide g of lead 3.00 g of sulfur g of lead sulfide 1.44 g of sulfur (leftovers) g of lead 1.56 g of sulfur g of lead sulfide 8.00 g of lead (leftovers) Ralph A. Burns, Fundamentals of Chemistry 1999, page 91

9 Daltons’ Models of Atoms Carbon dioxide, CO 2 Water, H 2 O Methane, CH 4

10 Dalton’s Atomic Theory 1. All matter is made of tiny indivisible particles called atoms. 2. Atoms of the same element are identical, those of different atoms are different. 3. Atoms of different elements combine in whole number ratios to form compounds 4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed. California WEB

11 Structure of Atoms Scientist began to wonder what an atom was like. Was it solid throughout with no internal structure or was it made up of smaller, subatomic particles? It was not until the late 1800’s that evidence became available that atoms were composed of smaller parts.

12 Radioactivity (1896) 1. rays or particles produced by unstable nuclei a. Alpha Rays – helium nucleus b. Beta Part. – high speed electron c. Gamma ray – high energy x-ray 2. Discovered by Becquerel – exposed photographic film 3. Further work by Curies Antoine-Henri Becquerel ( )

13 Thomson Model of the Atom J. J. Thomson - English physicist Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

14 Background Information Cathode Rays Form when high voltage is applied across electrodes in a partially evacuated tube. Originate at the cathode (negative electrode) and move to the anode (positive electrode) Carry energy and can do work Travel in straight lines in the absence of an external field

15 Cathode Ray Experiment 1897 Experimentation Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field. The rays bent towards the positive pole, indicating that they are negatively charged.

16 A Cathode Ray Tube Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 58

17 Thomson’s Experiment + - voltage source OFF ON + - By adding an electric field… he found that the moving pieces were negative.

18 J.J. Thomson He proved that atoms of any element can be made to emit tiny negative particles. From this he concluded that ALL atoms must contain these negative particles. He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson

19 William Thomson (Lord Kelvin) In 1910 proposed the Plum Pudding model –Negative electrons were embedded into a positively charged spherical cloud. Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 56 Spherical cloud of Positive charge Electrons

20 Thomson Model of the Atom J.J. Thomson discovered the electron and knew that electrons could be emitted from matter (1897). William Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere. The electrons were like currants in a plum pudding. This is called the ‘plum pudding’ model of the atom. - electrons

21 Other pieces Proton - positively charged pieces –1840 times heavier than the electron Neutron - no charge but the same mass as a proton. How were these pieces discovered? Where are the pieces?

22 Ernest Rutherford ( ) Learned physics in J.J. Thomson’ lab. Noticed that ‘alpha’ particles were sometime deflected by something in the air. Gold-foil experiment Rutherford PAPER Rutherford PAPER Animation by Raymond Chang Animation by Raymond Chang – All rights reserved.

23 Rutherford ‘Scattering’ In 1909 Rutherford undertook a series of experiments He fired  (alpha) particles at a very thin sample of gold foil According to the Thomson model the  particles would only be slightly deflected Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source particle source Lead collimator Gold foil  

24 Rutherford’s Apparatus beam of alpha particles radioactive substance gold foil circular ZnS - coated fluorescent screen Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120 Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.1908 Nobel Prize in Chemistry

25 Rutherford’s Apparatus Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120 beam of alpha particles radioactive substance fluorescent screen circular - ZnS coated gold foil

26 Lead block Polonium Gold Foil Florescent Screen California WEB

27 What He Expected The alpha particles to pass through without changing direction (very much) Because The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles California WEB

28 What he expected…

29 Because, he thought the mass was evenly distributed in the atom

30 What he got… richocheting alpha particles

31 The Predicted Result: expected path expected marks on screen mark on screen likely alpha particle path Observed Result:

32 Interpreting the Observed Deflections Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page gold foil deflected particle undeflected particles.. beam of alpha particles.

33 Density and the Atom Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. Small volume and big mass = big density This small dense positive area is the nucleus California WEB

34 Rutherford Scattering (cont.) Rutherford interpreted this result by suggesting that the  particles interacted with very small and heavy particles Particle bounces off of atom? Particle attracts to atom? Particle goes through atom? Particle path is altered as it passes through atom?. Case A Case B Case C Case D

35 Table: hypothetical description of alpha particles alpha rays don’t diffract alpha rays deflect towards a negatively charged plate and away from a positively charged plate alpha rays are deflected only slightly by an electric field; a cathode ray passing through the same field is deflected strongly... alpha radiation is a stream of particles... alpha particles have a positive charge... alpha particles either have much lower charge or much greater mass than electrons observation hypothesis (based on properties of alpha radiation) Copyright © by Fred SeneseFred Senese

36 Explanation of Alpha-Scattering Results Plum-pudding atom Alpha particles Nuclear atom Nucleus Thomson’s modelRutherford’s model

37 Interpreting the Observed Deflections Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page gold foil deflected particle undeflected particles.. beam of alpha particles.

38 Rutherford’s Gold-Leaf Experiment Conclusions: Atom is mostly empty space Nucleus has (+) charge Electrons float around nucleus Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120

39 Bohr’s Model Nucleus Electron Orbit Energy Levels

40 Bohr Model of Atom The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation. Increasing energy of orbits n = 1 n = 2 n = 3 A photon is emitted with energy E = hf e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

41 An unsatisfactory model for the hydrogen atom According to classical physics, light should be emitted as the electron circles the nucleus. A loss of energy would cause the electron to be drawn closer to the nucleus and eventually spiral into it. Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294

42 Niels Bohr In the Bohr Model (1913) the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. They are not confined to a planar orbit like the planets are.

43 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.


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