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Chapter 2: Atoms, Molecules and Ions

Chapter 2: Atoms, Molecules and Ions. Web Resources (Atomic Structure). Chem Team: Atomic Structure http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/AtomicStructure.html http://www.visionlearning.com/library/module_viewer.php?mid=50 Discovery of the electron

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Chapter 2: Atoms, Molecules and Ions

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  1. Chapter 2: Atoms, Moleculesand Ions

  2. Web Resources (Atomic Structure) • Chem Team: Atomic Structure • http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/AtomicStructure.html • http://www.visionlearning.com/library/module_viewer.php?mid=50 • Discovery of the electron • http://www.3rd1000.com/history/electrons.htm

  3. History of the Atomic Theory A. Democritus B. Aristotle C. Lavoisier D. Proust E. Dalton F. Modern Atomic Theory • History of Atomic Structure A. Thomson B. Milikan C. Rutherford D. Bohr E. Chadwick F. Quantum Atom • Subatomic Particles A. Atomic Number B. Mass Number and Isotopes C. Electrons and Ions D. Nuclear and Hyphenation Notation E. Average Atomic Mass • The Periodic Table • Naming and Formula Writing A. Formula Units B. Molecules C. Oxidation Number

  4. History of the Atomic Theory: Greeks • Original idea Ancient Greece (400 B.C..) • Democritus and Leucippus Greek philosophers

  5. History of the Atomic Theory: Greeks Democritus (400 BC) • Matter is made up of “atoms” • Atoms are always moving • Atoms join together • He was right • Problem: no evidence and not as popular • Coined the word: Atomos

  6. History of the Atomic Theory: Greeks Aristotle (350 BC) • All matter is continuous • Air  Water  Fire  Earth • He was very well known in his day • But he was completely wrong

  7. History of Atomic Theory: XVII-XVIII Centuries Boyle (1627-1691): defined the concept of an element. Did tremendous work with gases Newton(1642-1727) advocated the corpuscular( corpuscle – particle) theory of matter.

  8. History of Atomic Theory: Late XVIII Century • Combination Laws are formulated • Law of Conservation of Mass • Law of Definite Proportions • Law of Multiple proportions

  9. Law of Conservation of Mass Antoine Lavoisier (1770s) • Experiment: tin + oxygen  tin oxide mass before rxn = mass after rxn • Develops Law of Conservation of Mass • Matter cannot be created or destroyed (in a chemical or physical change)

  10. Law of Definite Composition Joseph Proust (1779) • In each compound elements combine in definite proportions to each other by mass • Example: carbon dioxide contains 27.3% carbon and 72.7% oxygen • Example: water, 11.2 % hydrogen and the rest oxygen

  11. Law of Multiple Proportions (Dalton) • If two elements form more that one compound, the ratio of one element that combines with 1 gram of the first element in each of the compounds is a small whole number. • Examples: 1.00 g of NO will combine with 1.14 g of O to form nitrogen monoxide, and 1.00 g of N will combine with 2.28 g of O to form nitrogen dioxide. The ratio of masses of oxygen in both compounds is 2.28: 1.14 = 2:1

  12. History of the Atomic Theory: Dalton • Dalton, unlike Democritus, collects evidence and develops: • Dalton’s Atomic Theory

  13. Dalton’s Atomic Theory: Postulates • All matter is made of tiny indivisible particles called atoms. • Atoms of the same element are identical in their properties, those of different elements are different. • Atoms of different elements combine in whole number ratios to form compounds • Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

  14. Explain the Combination Laws using Dalton’s Atomic Theory • Explain the combination laws using Dalton's Theory : check textbook.

  15. History of the Development of Modern Atomic Structure

  16. Discovery of Radioactivity • Becquerel: 1896. Discovered radioactivity • Maria Curie Sklodowska: discovered radium and polonium (1898)

  17. The Discovery of Electrons • J. J. Thomson(1856-1940) - English physicist. • Used cathode ray tube. • It is a vacuum tube - all the air has been pumped out. • Discovered the electron (1897) and the proton (1923) • Calculated the ratio between the charge of the electron and its mass: e/m

  18. Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

  19. Thomson’s Experiment Voltage source - + Vacuum Tube before the experiment: the tube is evacuated and power is about to be applied.

  20. Thomson’s Experiment Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  21. Thomson’s Experiment Voltage source - + • Passing an electric current makes a beam appear to move from the negative to the positive end in straight line

  22. Thomson’s Experiment Voltage source • When external electric field (outside the tube) is added:

  23. Thomson’s Experiment Voltage source + anode - cathode External electric field External electrodes

  24. Thomson’s Experiment Voltage source + - • By adding an external electric field

  25. Thomson’s Experiment Voltage source + - • By adding an external electric fieldhe found that the moving pieces were negative: they bend towards the positive anode.

  26. Thomson’s Model • Thomson proposed the following: • the atom was like plum pudding • Ball of positively charged matter • electrons scattered throughout and can be easily removed

  27. Millikan’s Oil Drop Experiments Robert Milikan (1909) • Oil Drop Experiment • Measured the electrical charge on the electron • Mass can be calculated (Thomson determined the e/m ratio) • Mass is 1/1840 the mass of a hydrogen atom • electronhas a mass of 9.11 x 10-28 g

  28. Oil-Drop Experiment: Animation • http://wps.prenhall.com/wps/media/objects/602/616761/MillikanOilDropExperiment.html • http://www.britannica.com/nobel/cap/omillik001a4.html • http://www.daedalon.com/oildrop.html

  29. Discovery of the Nucleus • So, at this point we know: • Dalton’s Atomic Theory • Electrons are negatively charged (Thomson) • Atoms are neutral • The mass of an electron is very small (Millikan) • Where is the mass of the atom?

  30. Discovery of the Nucleus: Rutherford Ernest Rutherford (1909): Gold Foil Experiment • Alpha particles (+2 charge) beamed at thin gold foil • Progress followed on a scintillating screen • When alpha particles (+2) hit screen, the screen lights up • Most particles went through • Some particles bounced to the sides and at 180º • In process: discovered the proton • Proton is positively charged

  31. Florescent Screen Lead block Uranium Gold Foil THE SET-UP

  32. Rutherford Expected • The alpha particles to pass through without changing direction Because • The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

  33. Because

  34. Because, he thought the mass was evenly distributed in the atom

  35. Because, he thought the mass was evenly distributed in the atom

  36. Fluorescent Screen Lead block Uranium Gold Foil

  37. Rutherford’s results

  38. Rutherford’s Gold Foil Experiment Here’s what it looked like (pg 72) • http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

  39. Discovery of the Nucleus: Rutherford • Discovered the: • Proton: positively charged piece 1840 times heavier than the electron • Predicted the existence of • Neutron: no charge but the same mass as a proton. • Where are the pieces?

  40. Discovery of the Nucleus: Density of the Atom • Since most of the particles went through, the atom is mostly empty. • Since some of the alpha paricles were strongly deflected, the positive pieces were heavy. • Small volume, big mass, big density • This small dense positive area is the nucleus

  41. Rutherford: Explanation + Atom is: • mostly empty • small dense, positive piece at center: nucleus • Alpha particles are deflected by it if they get close enough

  42. Rutherford: Explanation +

  43. Rutherford’s Contributions • Discovered the nucleus. • Did not know where to place the electrons • Proposed the NUCLEAR MODEL of the atom also called the PLANETARY ATOM • Proposed the existence of the neutron • Determined the size of the nucleus (1x10-15 m) and the atom (1 x 10-11 m for hydrogen atom, larger for other atoms) • Elucidated the structure of the alpha-particles • Performed the first nuclear reaction

  44. Discovery of the Nucleus • At this point in 1909, we know: • p+ = 1.67 x 10-24 g • e- = 9.11 x 10-28 g • The charges are balance! • Size of atom: 1x 10-11m (for hydrogen atom, the smallest in the periodic table) • Size of nucleus: 1 x 10-15 m • But, • How are the electrons arranged? • There is still mass that is unaccounted for

  45. Structure of the Atoms: Niels Bohr Niels Bohr (1913) • Electrons orbit nucleus in predictable paths • Much more on him later

  46. Discovery of the Neutron: Chadwick Chadwick (1935) • Discovers neutron in nucleus • Neutron is neutral • n0 • Mass is 1.67 x 10-24 g

  47. Structure of the Atom: 1913 • Charges balanced (protons in nucleus + electrons) • Mass accounted for (protons + neutrons) • Placement of electrons: uncertain • Today, we subscribe to the Quantum Atom Theory to describe the atomic structure

  48. Quantum Atom Quantum Atom Theory • The atom is mostly empty space • Two regions: • Nucleus- protons and neutrons • Electron cloud- region where you have a 90% chance of finding an electron

  49. History of the Atomic Theory: Summary

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