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History of the Atom Leading up to the current model

History of the Atom Leading up to the current model. Models of the Atom. Earliest Models: Dalton (and contemporaries) “Billiard Ball” Solid sphere Cannot be divided up into smaller particles or pieces. Atom neutral and no charge Atoms of same element are made of the same types of atoms.

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History of the Atom Leading up to the current model

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  1. History of the Atom Leading up to the current model

  2. Models of the Atom Earliest Models: Dalton (and contemporaries) “Billiard Ball” Solid sphere Cannot be divided up into smaller particles or pieces. Atom neutral and no charge Atoms of same element are made of the same types of atoms. Faraday Suggested that structure of atom is somehow related to electricity. Long series of experiments: Atoms contain particles that have electrical charge.

  3. Models of the Atom (cont.) Faraday (cont.) Greeks knew that if you rubbed amber with a cloth, it attracted dust or other particles. Static electricity Franklin Studied Static Electricity Famous Kite Flying experiment Findings: Object could have one of two electrical charges Called them positive and negative (+, -) Alike charges repel (+ +) and (- -) Lightning was static electricity on a larger scale.

  4. Models of the Atom (cont.) Others (mid-1800s) Scientists investigated electric currents Cathode Ray Tube (CRT) J.J. Thompson (1896) Systematic studies on cathode rays Movie concluded that: Through experiments, cathode rays were composed of negative particles and these negative particles could be manipulated with magnet & electric currents. Atoms were not indivisible, solid sphere. But had substructure(s). Eventually, addition of “pinwheel” to the tube showed that particles in beam had mass. Uses of this Technology

  5. Models of the Atom (cont.) J.J. Thompson (cont.) called negative particles “Electrons” Was able to determine ratio of electron’s electrical charge to mass (1.76 x 108 coulombs per gram) Millikan (1909) Oil Drop Experiment (Movie) Measured the charge of an electron charged droplets with x-rays (negative charge) Varied rate of falling by changing charge in two charged plates. Calculated that the charge on each oil drop was a multiple of 1.60 x 10-19 C (coulombs) Figured the charge for an electron must be 1.60 x 10-19 From this and Thompson’s ratio, calculated the mass of e-.

  6. Models of the Atom (cont.) What did this do to the idea of the Model? This new research/data showed that the atom was NOT a solid sphere. But had “parts” that had a negative charge. However, the overall atom was neutral in charge. Therefore, there must be positive “parts” to balance the negative “parts”. Gave rise to the “Plum Pudding” Model. Electrons (-) spread randomly throughout the atom. And surrounded by randomly spread out positive (+) parts.

  7. Models of the Atom (cont.) Radioactivity Becquerel (1896) Accidentally discovered uranium sample was radioactive (placed on photography film). Radioactivity: Spontaneous emission from atom Curie (Marie and hubby Pierre) Becquerel’s colleagues Isolated 2 other radioactive materials Radium and Polonium

  8. Models of the Atom (cont.) Radioactivity Scientists soon after discovered several things: Radioactivity accompanies fundamental changes in an atom. A chemical change happens as radiation is given off!!! Rutherford (early 1900s) Studied Radioactivity Used 2 electrically charged plates (Movie) Found that some radiation deflected towards negative plate. Called it an ALPHA RADIATION. Other radiation deflected towards positive plate. Called it BETA RADIATION. Later scientists found another radiation which was undeflected by electrical charge. GAMMA RADIATION. Both Alpha and Beta radiations were shown to be particles.

  9. Models of the Atom (cont.) Rutherford (cont.) Concluded that atoms contain electrons, but were electronically neutral Alpha-scattering experiment (Gold Foil Experiment) Movie

  10. Models of the Atom (cont.) Gold Foil Experiment (results) A few particles were deflected off their path. Some particles “bounced back” Think of playing pool. Glancing hits vs. more direct hits. Findings. Most of the mass was concentrated in the core. The positive charge was concentrated in the core. Most of the area around an atom was “empty”. Nucleus is small compared to the atom, but very large compared to electrons. Rutherford called this core, the nucleus. Lead to new model: Nuclear Model

  11. Rutherford (cont.) Electrons around the atom’s nucleus.

  12. Moseley Student of Rutherford’s Found that atoms of each element contained unique positive charge in their nucleus. Helped solve the mystery of what makes atoms of one element different from another element. Atomic Number = # of PROTONS Proton is the positive charge within an atom’s nucleus. Atoms are electronically neutral. # of Protons (atomic #) = # of Electrons

  13. Bohr (jump in time): Visualized the atom like the solar system. Electrons in distinct “orbits” around the atom’s nucleus.

  14. Eventually, scientists came up with: Electron Cloud Model Electrons are located within certain 3-D regions around the nucleus called clouds.

  15. Atoms The practical “stuff”

  16. Atoms Topic 2: Atomic structure (4 hours) 2.1The atom 1 hour Assessment statement 2.1.1 State the position of protons, neutrons and electrons in the atom. 2.1.2 State the relative masses and relative charges of protons, neutrons and electrons. 2.1.3 Define the terms mass number (A), atomic number (Z) and isotopes of an element. 2.1.4 Deduce the symbol for an isotope given its mass number and atomic number. 2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number, atomic number and charge. 2.1.6 Compare the properties of the isotopes of an element. 2.1.7 Discuss the uses of radioisotopes Examples should include 14C in radiocarbon dating, 60Co in radiotherapy, and 131I and 125I as medical tracers.

  17. Atoms Made up of: Electrons Negative charge Around the nucleus VERY small mass Protons Positive charge In the nucleus Accounts for large part of mass of an atom Neutrons No charge In the nucleus Accounts for large part of mass of an atom. “Glue” of an atom

  18. Atoms (cont.) Ions Atoms can gain or lose electrons Become Ions (more or less electrons than the # of protons) Can be either POSITIVE or NEGATIVE Charge of ion = # of Protons - # of Electrons e.g. Magnesium (Mg) atomic # 12. Loses 2 e- # of Protons 12 -# of electrons -10 +2 Mg2+ Try a few: Ca loses 2 e- b) F gains 1 e- c) As gains 3 e- Ca2+ F1- As3-

  19. Atoms (cont.) Isotopes All about Neutrons “Glue” Mass is 1 a.m.u. (Atomic Mass Unit) (Mass of Proton are each 1 a.m.u.) Think about the charges in the nucleus (Repelling + charges) More glue is needed as the # of protons climbs Same # of protons (why?) and different # of neutrons e.g. Hydrogen H has 1 proton, 0 neutrons. Mass is 1 a.m.u. Deuterium (2H) has 1 proton, 1 neutron. Mass is 2 a.m.u. Tritium (3H) has 1 proton, 2 neutron. Mass is 3 a.m.u. Mass Number: Sum of isotope’s protons and neutrons.

  20. Isotopes (cont.) Mass # 37Cl Atomic # 17 What is that number (decimal) at the bottom under the symbol? WHY? Average of the isotope’s mass e.g. 12C 98.90% (of mass # 12) 13C 1.10 % (of mass # 13) Average atomic mass: 12.011 amu

  21. Radioactivity

  22. Changes in the Nucleus Radioactivity Nuclear stability (instability) Recall: Protons (? Charge) Therefore: REPEL each other Neutron (? Charge) “Glue” to hold Protons together Strong Nuclear Force (the glue)

  23. Nuclear stability (cont.) From 1-20, approximate 1:1 Protons: Neutrons. Beyond 20, more Neutrons. 83 and beyond, spontaneous emissions Can’t hold together indefinitely Falls apart Called “Decay” Not only too little “glue”, but also too much. As a general rule, lighter & heavier isotopes (vs. common isotope) are likely radioactive.

  24. Types of Radioactive Decay (basics) Alpha Beta Gamma Specials Positron Emission Electron Capture Alpha Consists of 2 protons & 2 neutrons What is that? 42He or 42a Helium Nucleus (no electrons) Penetration power: Stopped by paper Charge/Mass: 2+ / 4 amu With Alpha, think LOSS

  25. Types of Radioactive Decay (cont.) Beta Consists of high speed electrons What is that? e- or 0-1b Where do they come from???? Penetration power: Stopped by heavy clothing Charge/Mass: 1- / ~0 amu With Beta, think CHANGE

  26. Types of Radioactive Decay (cont.) Gamma Consists of high energy photons What is that? g Similar to X-rays Penetration power: Stopped by lead, concrete Charge/Mass: 0 / 0 With Gamma, Think ENERGY See Radiation Movie

  27. Types of Radioactive Decay (cont.) Special Types: Positron Emission (also called Beta positive decay) A positron is exactly like an electron in mass and charge force except with a positive charge. Charge/Mass: 1+ / 0 amu It is formed when a proton breaks into a neutron with mass and no charge = positron (no mass and the positive charge) Positron emission is most common in lighter elements with a low neutron to proton ratio.

  28. Special Types (cont.) Electron Capture A captured electron joins with a proton in the nucleus to form (change to) a neutron. = one less proton, turned into a neutron. Charge/Mass: changes from +  0 / same Electron capture is common in larger elements with a low neutron to proton ratio.

  29. Decay of Uranium

  30. Uranium

  31. Reminder: Nuclear Equations Equation that keeps track of the reaction’s components. Alpha decay of Gold 18579Au  18177Ir + 42a Decay of Iodine 13153I  13154Xe + 0-1b Try a few (solve): 23892U  23490Th + ______ 2411Na  ______ + 0-1b

  32. Uranium: Enriched Uranium: U-235 Low: 3-4% U-235 (remaining is U-238) Reactor Grade High: 90% U-235 (remaining is U-238) Weapons Grade Slightly (0.9%-2%) (replaces natural U in some reactors Recovered: less U-235 than in natural occurring Uranium Depleted Uranium: U-238 Remnants after enrichment Less radioactive then natural Uranium VERY Dense -Useful for armor and penetrating weapons NOTE: Depleted U-238 is still radioactive. Just LESS.

  33. Uses for Radioactive materials: Weapons: Nuclear weapons “Little Boy” -Uranium, gun-type -City of Hiroshima on August 6, 1945 “Fat Man” -Plutonium, Implosion -City of Nagasaki on August 9, 1945

  34. Uses for Radioactive materials: Smoke Detectors: Am-241 Gives off a particles Ionizing energy (makes Ions) Ionize smoke particles Allows completion of a circuit (allows electricity to flow) With a complete circuit, alarm sounds. Cancer Treatment (BNCT) Boron Neutron Capture Therapy Patient is given Boron-10 (10B) Using a neutron beam, doctors create thermal neutrons which changes Boron-10 into excited Boron-11 Boron-11 decays, given off an a particle a particle penetrated one-two cells deep, Kills the cell(s)

  35. Uses for Radioactive materials: Cobalt-60 Used to sterilizes hospital equipment (by gamma irradiation). Used as a radiation source in radiotherapy (cancer treatment). Ionizing radiation (makes ions by knocking e- off). Damages genetic material, keeps cell from growing. Iodine-131 Used as a medical tracer b and g radiation emitter Investigate thyroid gland activity Diagnose and treat thyroid cancer Short half-life (8 days) Quickly eliminated from body Iodine-125 Used for treatment of prostate cancer 80 day half-life (b emitter)

  36. Uses for Radioactive materials: Mineral (geological). Compare the amount of U-238 to Pb-206. Compare amount of K-40 to Ar-40. Carbon-14 dating The radioactive C-14 method of dating is used to determine the age of organic matter that is several hundred years to approximately 50,000 yrs old. C-14 is continually formed in nature by the interaction of neutrons with N-14 in the Earth’s atmosphere. The neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere. C-14, along with non-radioactive C-13 and C-12, is converted into CO2 and assimilated by plants and organisms. C-14/C-12 ratio consistent until cell death When plant or animal dies, it no longer acquires carbon. C-14 begins to decay.

  37. Radiocarbon DatingC-14 Half-Life = 5730 Years

  38. Nucleosynthesis: How were/are elements formed?

  39. Nucleosynthesis (cont.): How were/are heavier elements formed? Fusion with increasingly larger and larger elements 42He + 42He  84Be + g 42He + 84Be 126C + g Elements present in stars (depends on its size) H, He, C, O, Ne, Mg + other heavier elements Larger stars (greater gravitational energies), heavier elements Each “layer” acts to fuel the next “layer”. H  He; He  C; C  O; O  Ne; Ne  Mg; Mg  Si; Si  Fe Heavier elements are created in supernovas (exploding stars).

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