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Atomic Structure

Explore the historical development of the atom from the ancient Greeks' elemental theory to John Dalton's foundational Atomic Theory in 400 B.C. to the late 1700s. Discover the evolution of ideas and experiments that led to our modern understanding of atoms and their properties.

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Atomic Structure

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  1. Atomic Structure Chapter 4

  2. Development of the Atom

  3. ~ ~ Fire Water Earth Air The Hellenic Market

  4. Not the history of atom, but the idea of the atom In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and water. Democritus and Leucippus Greek philosophers ~ ~ The GreeksHistory of the Atom

  5. Mental Experiment – Atoms Exist • Looked at beach • Made of sand • Cut sand - smaller sand • Smallest possible piece? • Atomos - not to be cut

  6. Greek Model Democritus • Greek philosopher • Idea of ‘democracy’ • Idea of ‘atomos’ • Atomos = ‘indivisible’ • ‘Atom’ is derived • No experiments to support idea Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE

  7. FIRE Hot Dry ‘MATTER’ AIR EARTH Wet Cold WATER Four Element Theory • Plato was an atomist • Thought all matter was composed of 4 elements: • Earth (cool, heavy) • Water (wet) • Fire (hot) • Air (light) • Ether (close to heaven) Relation of the four elements and the four qualities Blend these “elements” in different proportions to get all substances

  8. Anaxagoras (Greek, born 500 B.C.) Suggested every substance had its own kind of “seeds” that clustered together to make the substance, much as our atoms cluster to make molecules. Empedocles (Greek, born in Sicily, 490 B.C.) Suggested there were only four basic seeds – earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything. Democritus (Greek, born 460 B.C.) Actually proposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years. Aristotle (Greek, born 384 B.C.) Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements which combined as shown in the diagram (previous page). Hot + dry made fire; hot + wet made air, and so on. Some Early Ideas on Matter O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles1982, page 26,

  9. Who Was Right? • Greek society was slave based • Beneath famous to work with hands • did not experiment • Greeks settled disagreements by argument • Aristotle was more famous • He won! • His ideas carried through middle ages. • Alchemists change lead to gold California WEB

  10. Alchemy • After that chemistry was ruled by alchemy. • They believed that they could take any cheap metals and turn them into gold. • Alchemists were almost like magicians. • Elixirs, physical immortality

  11. GOLD SILVER COPPER IRON SAND . . . . . . . . . . . . . . . Alchemy Alchemical symbols for substances… transmutation: changing one substance into another D In ordinary chemistry, we cannot transmute elements.

  12. Contributions of alchemists: • Information about elements • - the elements mercury, sulfur, and antimony were discovered • - properties of some elements • Develop lab apparatus / procedures / experimental techniques • - alchemists learned how to prepare acids. • - developed several alloys • - new glassware

  13. Alchemists’ Contributions • The elements mercury, sulfur, and antimony were discovered. • Alchemists learned how to prepare acids. • Develop lab apparatus / procedures • How to make some alloys • Properties of some elements

  14. Timeline Greeks (Democratus ~450 BC) Discontinuous theory of matter Issac Newton (1642 - 1727) ALCHEMY 400 BC 300 AD 1000 2000 Greeks (Aristotle ~350 BC)) Continuous theory of matter American Independence (1776)

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

  16. Foundations of Atomic Theory • Law of Conservation of Mass • Mass is neither destroyed nor created during ordinary chemical reactions. • 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.

  17. H H O O H2 H2O H H O2 + O H H H2 O H H2O H Conservation of Atoms John Dalton 2 H2 + O2 2 H2O 4 atoms hydrogen 2 atoms oxygen 4 atoms hydrogen 2 atoms oxygen Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

  18. H H O O H2 H2O H H O2 + O H H H2 O H H2O H Legos are Similar to Atoms Legos can be taken apart and built into many different things. Atoms can be rearranged into different substances.

  19. Law of Multiple ProportionsJohn Dalton (1766 – 1844) If two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number. e.g. H2O & H2O2 water hydrogen peroxide Ratio of oxygen is 1:2 (an exact ratio)

  20. Daltons Atomic Theory • Dalton stated that elements consisted of tiny particles called atoms • He also called the elements pure substances because all atoms of an element were identical and that in particular they had the same mass.

  21. Dalton’s Theory Continued • He also said the reason why elements differed from one another was that atoms of each element had different masses. • He also said that compounds consisted of atoms of different elements combined together. • Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

  22. Dalton’s Symbols John Dalton 1808

  23. Carbon dioxide, CO2 Methane, CH4 Water, H2O Daltons’ Models of Atoms

  24. 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

  25. Dalton’s Atomic Theory • All matter consists of tiny particles. • Dalton, like the Greeks, called these particles “atoms”. • Atoms of one element can neither be subdivided nor changed into atoms of any other element. • Atoms can neither be created nor destroyed. 4. All atoms of the same element are identical in mass, size, and other properties. • Atoms of one element differ in mass and other properties from atoms of other elements. • In compounds, atoms of different elements combine in simple, whole number ratios.

  26. 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.

  27. Thomson Model of the Atom J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

  28. Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Metal plate A Cathode Ray Tube Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 58

  29. A Cathode Ray Tube Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 58

  30. Thomson’s Experiment voltage source - + vacuum tube metal disks

  31. Thomson’s Experiment voltage source - + vacuum tube metal disks

  32. Thomson’s Experiment voltage source ON - OFF + Passing an electric current makes a beam appear to move from the negative to the positive end

  33. Thomson’s Experiment voltage source ON - OFF +

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

  35. The Effect of an Electric Field on Cathode Rays Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 118

  36. 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 something balancing the negative charge. J.J. Thomson

  37. Spherical cloud of Positive charge Electrons Thomson • In 1910 proposed the Plum Pudding model • Negative electrons were embedded into a positively charged spherical cloud. Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 56

  38. Plum-Pudding Model Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 56

  39. 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

  40. - - - - - Thomson’s Model • Found the electron • Couldn’t find positive (for a while) • Said the atom was like plum pudding • A bunch of positive stuff, with the electrons able to be removed

  41. Rutherford Model of the Atom

  42. Ernest Rutherford (1871-1937) • Learned physics in J.J. Thomson’ lab. • Noticed that ‘alpha’ particles were sometime deflected by something in the air. • Gold-foil experiment

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

  44. Lead collimator Gold foil a particle source q Rutherford ‘Scattering’ • In 1909 Rutherford undertook a series of experiments • He fired a (alpha) particles at a very thin sample of gold foil • According to the Thomson model the a 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

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

  46. Rutherford’s Experiment Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 56

  47. 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

  48. What he expected… California WEB

  49. What he got… California WEB

  50. . . . . . . . . . . . . . . gold foil Interpreting the Observed Deflections . beam of alpha particles undeflected particles . . deflected particle Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

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