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Chapter 4 (Partial) Structure of the Atom

Chapter 4 (Partial) Structure of the Atom. 4.1 Early Theories of Matter (& Early Chemistry – Alchemy and Related) 4.2 Subatomic Particles & Nuclear Atom 4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book). Section 4.1 Early Ideas About Matter.

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Chapter 4 (Partial) Structure of the Atom

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  1. Chapter 4 (Partial)Structure of the Atom 4.1 Early Theories of Matter (& Early Chemistry – Alchemy and Related) 4.2 Subatomic Particles & Nuclear Atom 4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

  2. Section 4.1 Early Ideas About Matter The ancient Greeks tried to explain matter, but the scientific study of the atom began with John Dalton in the early 1800's. • Compare and contrastthe atomic models of Democritus, Aristotle, and Dalton. • Describe the activities related to the chemical sciences that occurred between the time of Aristotle and the early 19th century when Dalton’s theory was published. • List the components of Dalton’s atomic theory. • Explainhow Dalton's theory explains the conservation of mass.

  3. Section 4.1 Early Ideas About Matter (Cont.) • Identify the components of Dalton’s theory that are not strictly correct and provide examples of why they aren’t. • Name the two instruments that are routinely used to obtain images of atoms. • Describe the basic operational principles of the Scanning Tunneling Microscope (STM).

  4. Section 4.1 Early Ideas About Matter Key Concepts • Democritus was the first person to propose the existence of atoms. • According to Democritus, atoms are solid, homogeneous, and indivisible. • Aristotle did not believe in the existence of atoms. • John Dalton’s atomic theory is based on numerous scientific experiments. • The scanning tunneling microscope (STM) and the modified scanning transmission electron microscope (modified STEM) are instruments capable of atomic scale imaging.

  5. Early Philosophers • Thought matter formed of: • Earth • Air • Fire • Water

  6. - 5 0 0 - 2 5 0 0 2 5 0 5 0 0 7 5 0 1 0 0 0 1 2 5 0 1 5 0 0 1 7 5 0 2 0 0 0 Atomic Theory Timeline • History: Development of Atomic Model Democritus Aristotle Boyle Lavoisier Dalton J. Dalton J. Proust A. Lavoisier R. Boyle Empedocles Aristotle Democritus Leucippus Zeno R. Bacon - 5 0 0 - 2 5 0 0 2 5 0 5 0 0 7 5 0 1 0 0 0 1 2 5 0 1 5 0 0 1 7 5 0 2 0 0 0

  7. Democritus Greek, 460-370 BC • First to propose matter was not infinitely divisible = concept of atom

  8. Democritus, Greek Philosopher (460-370 BC) • His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained • This piece would be indivisible • Named the smallest piece of matter “atomos,” meaning “not to be cut”

  9. Democritus – Atomic Theory To him, atoms were small, hard particles that were all made of same material but were different shapes and sizes Atoms were infinite in number, surrounded by empty space, and always moving and capable of joining together

  10. Democritus’ Concept of Matter • Matter is empty space through which atoms move • Atoms are solid, homogeneous, indestructible, indivisible • Different kinds of atoms have different sizes and shapes • Differing properties of matter are due to atoms size, shape, & movement • Changes in matter result from changes ingroupings of atoms and not changes in atoms themselves

  11. Aristotle Greek Philosopher (384-322 BC) • 14 years old when Democritus died • Believed matter made of 4 basic elements (earth, air, fire and water) • Disagreed with Democritus - believed matter was continuous (did not accept idea of the “void”) • His ideas endured for 2000 yrs

  12. Alchemy for 2000 years • Aristotle believed any substance could be transmuted (transformed) into any other substance simply by changing relative proportions of the 4 basic qualities • This mindset dominated quest for new substances done by the alchemists

  13. Alchemy for 2000 years During the search for ability to transmute matter (e.g., change lead into gold), they did a lot of good experimentation that laid foundation for modern science • Idea of transmutation laid foundation for alchemy • Alchemists were searching for evolution from ignorance to enlightenment by searching for: • elixir of life (source of eternal life/youth) • philosopher’s stone (substance to turn base metals into gold; also el. of life) • aqua vitae (“water of life” – concentrated ethanol solution – whiskey etc) • panacea (substance meant to cure all diseases)

  14. Alchemy • "The hopeless pursuit of the practical transmutation of metals was responsible for almost the whole of the development of chemical technique before the seventeenth century, and further led to the discovery of many important materials.”

  15. Alchemy • Popular belief is that Alchemists made contributions to the "chemical" industries of the day—ore testing and refining, metalworking, production of gunpowder, ink, dyes, paints, cosmetics, leather tanning, ceramics, glass manufacture, preparation of extracts, liquors, and so on • Alchemists contributed distillation to Western Europe

  16. Science During 1600’s to1800’s • Scientists were discovering concepts and relationships by doing large, basic experiments with stoves, pots, ovens, and basic glassware, much of which had been developed by alchemists • With observable properties came explanations!

  17. Robert Boyle 1627-1691 • Sometimes referred to as Father of Modern Chemistry • One of first to publish all experimental details of his work, including experiments that did not work • Boyle revived Democritus’ ideas by proposing that a substance was not element if it were made of two or more components

  18. Robert Boyle ~ 1660 • Best known for his quantitative work with gases (Boyle’s Law) • Still believed in alchemy – that metals could be converted into gold • Was first to propose existence of elements in the modern sense • Boyle considered a substance to be an element unless it can be broken down into simpler substances

  19. Marie-Anne and Antoine Lavoisier 1743-1794 • Mother and father of modern chemistry? • Studied various types of reactions involving oxygen: respiration, burning, rusting

  20. Antoine Lavoisier (France) ~1760 • Studied chemical reactions quantitatively • Credited with being first to propose law of conservation of matter

  21. Lavoisier • Was sure that air contained > one element • Was able to determine amount of “reacting component” in air - named this component oxygen

  22. Lavoisier • Pictured experiment demonstrates Law of Conservation of Mass

  23. Lavoisier Law of Conservation of Mass • There is no detectable change in total mass of materials when they react chemically to form new materials • Mass of products will equal mass of reactants in a chemical reaction • During chemical reaction, matter neither created nor destroyed

  24. Joseph Proust (France, 1754-1826) • ~1794 Studied chemical composition of compound copper carbonate (CuCO3) • Found all samples of CuCO3 had same relative composition of elements by mass: 5.3 parts Copper: 4 parts Oxygen: 1 part Carbon • This finding led to law of definite proportion

  25. John Dalton (1766-1844) • A schoolteacher! • Devised Law of Multiple Proportions “when two elements form more than one compound, they come together in whole number ratios”

  26. John Dalton (1766-1844) • Used work of Lavoisier, Proust, and Gay-Lussac to revive Democritus’ idea that matter was made of atoms • Based much of his theory on • Law of Conservation of Mass • Law of Constant Composition

  27. Matter made up of atoms. Atoms of given element identical. Atoms can’t be created, destroyed or divided. Atoms may combine in the ratio of small, whole numbers to form compounds. In chemical reactions, atoms are separated, combined, or rearranged. All atoms of one element have the same mass. Atoms of two different elements have different masses. John Dalton’s Atomic Theory

  28. John Dalton’s Atomic Theory • Matter composed of extremely small atoms • Atoms of given element are identical • Atoms of different elements are different • Can’t be created, divided, or destroyed • Different atoms combine in whole number ratios to form compounds • In chemical reactions, atoms are separated, combined or rearranged

  29. Compound AB2 Element A Element B mass = mA + mB mass = mA mass = mB Dalton’s Atomic Theory • Experimental evidence • looked at mass ratios of compounds • Theory explained conservation of mass

  30. Dalton’s Atomic Theory • Slightly wrong about • Indivisibility of atoms (subatomic particles) • All atoms of same element having identical properties (isotopes) • Although atoms themselves not created or destroyed, slight changes in mass occur as energy absorbed/released (thanks to James Kong & A Einstein) • “Exotic” matter (neutron stars, plasmas, dark matter, etc) not composed of atoms as such (thanks to Adam Sorrentino)

  31. Atom Definition • Smallest particle of an element that retains the property of the element • This simple definition does not deal with the reality uncovered by modern nanotechnology research – individual atoms or small clusters of atoms of an element do not always behave in the same way as a bulk sample of the element

  32. Imaging Atoms • Atom diameters ~ 0.1 to 0.5 nm (water molecule diameter ~0.3 nm) • Techniques exist to “image” atoms (not really “seeing” them in the conventional sense of the word) • Not readily available until STM commercialized (see following)

  33. Schematic of STM

  34. STM Operation • Based on “tunneling current” • Starts to flow when sharp tip approaches conducting surface at distance of ~ 1 nm • Current extremely sensitive to distance • Tip mounted on a piezoelectric tube • Allows tiny movements by applying a voltage at its electrodes

  35. STM Electronics control tip position so tunneling current (tip-surface distance) is kept constant while scanning a small area of the sample Movement recorded - displayed as an image of the surface topography Under ideal circumstances individual atoms of a surface can be resolved

  36. STM – Moving Atoms Modified STM can be used as a tool for picking up, moving, and putting down atoms

  37. Imaging Atoms: Modified Scanning Transmission Electron Microscope • In 2002, IBM researchers and their collaborators modified an electron microscope; allowed clear images at the atomic scale to be made • Modified electron microscope is second major instrument to provide images of atoms • Can’t be used to move atoms like STM type instruments

  38. Practice • Early & current theories of matter • Problems 1- 5, page 91 • Problems 29 – 33, page 112

  39. Chapter 4 (Partial)Structure of the Atom • 4.1 Early Theories of Matter • 4.2 Subatomic Particles & Nuclear Atom • 4.2.5 Ultimate Structure of Matter – The Standard Model (Not in Book)

  40. Section 4.2 Defining the Atom An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus. • Define atom. • Distinguish between the subatomic particles in terms of relative charge and mass. • Describe the structure of the atom, including the locations of the subatomic particles and the relative sizes of the atom and the nucleus. • Identify the scientists that contributed to the discovery of the nature of the atom and be able to describe their specific contribution and the experiment on which their discovery was based.

  41. Section 4.2 Defining the Atom Key Concepts • An atom is the smallest particle of an element that maintains the properties of that element. • Electrons have a 1– charge, protons have a 1+ charge, and neutrons have no charge. • An atom consists mostly of empty space surrounding the nucleus; the size of the atom relative to the size is the nucleus is about 10,000.

  42. Crookes (Cathode Ray) TubeSee page 92, Figure 4-7

  43. Effect of Electric and/or Magnetic Fields on Electron Trajectory

  44. J J Thomson Discovering the Electron • From cathode ray tube experiments, it was determined that rays: • Were actually stream of charged particles • Carried negative charge

  45. Discovering the Electron • Thomson (1856-1940) • Measured effect of electric and magnetic fields on cathode ray to determine ratio of charge to mass (q/m) for electron • From comparison with known (q/m) values, concluded that electron mass much less than hydrogen atom  must be a subatomic particle • Did not determine actual value of mass

  46. Discovering the Electron • Millikan (1868-1953) • Determined charge on electron • from oil drop experiment (see following) • From mass/charge ratio (previously determined by Thomson), calculated electron mass, me me =1/1840 mass of hydrogen atom

  47. Millikan’s Oil Drop Experiment • Ions produced by energetic radiation (X-rays) • Some ions attach to oil droplets, giving them a net charge • Fall of droplet in electric field between the condenser plates is speeded up or slowed down, depending on the magnitude and sign of charge on droplet

  48. Millikan’s Oil Drop Experiment Atomizer Electrically charged condenser plates

  49. Millikan’s Oil Drop Experiment • Analyzed data from a large number of droplets • Concluded that the magnitude of charge (q) on a droplet is an integral multiple of electronic charge (e) • q = ne • (where n = 1, 2, 3, . . . ).

  50. Plum Pudding Atomic Model AKA “chocolate chip cookie dough” model Proposed by Thomson Smeared out “pudding” of positive charge with negative electron “plums” imbedded in it + + + Electrons (negative)

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