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Atomic Structure Early Atomic Theory Democritus (400 BC) suggested that the world was made of 2 thing empty space and tiny, invisible particles called atoms .

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Unit 2 - Nuclear Chemistry and the Structure of the Atom

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  • 1. Atomic Structure Early Atomic TheoryDemocritus (400 BC) suggested that the world was made of 2 thing empty space and tiny, invisible particles calledatoms .

2. Aristotle Aristotle believed matter was composed of 4 basic elements, water, fire, earth, and air.He was a well respected so everyone discounted Democrituss atomic theory and believed Aristotles theory without question. 3. John Dalton(early 1800s) using the experimental observations of others, Lavoisier, and Proust proposed his own atomic theory. 4. Daltons Atomic Theory

  • All elements are composed oftiny
  • particles called atoms.
  • Atoms of the same element areidentical . The atoms of any one element are different from those of another element.
  • Isotopesof an element are not identical .

5.

  • Atoms cannot bedivided ,
  • created, or destroyed.Atoms are divisible by anuclearcharge.
  • Atoms of different elements can combine with one another in simple, whole number ratios to formcompounds .
  • Chemical reactions occur when atoms arejoined ,separated , orrearranged .However , atoms of one element arenotchanged into atoms of another element.

6. Early Research on Atomic Particles Thomson(1867) used a cathode ray tube to prove the existence ofnegativelycharged subatomic particles calledelectrons . 7. Cathode Rays and the Electron Thomson investigated cathode rays using a devise called a cathode ray tube or CRT.Currents of electricity were pumped into vacuum tubes causing the tube to fluoresce.Thomason used magnets to determine the identity of the particles making up the rays.He found them to be tiny, negatively charged particles. 8. 9. Robert Millikan (1909) used his oil drop experiment to prove that the charge on of an electron is-1. 10. 11. Rutherford Using his gold foil experiment, Rutherford proposed that the atoms is composed of a lot ofempty space with, a small dense,positivelycharged nucleus. 12. 13. Thomson used a modified cathode ray tube to prove the existence of a positively charged subatomic particle called aproton .It has equal butoppositecharge to the electron(+1)and a mass1840times heavier than an electron. 14. Chadwick Chadwick used a device to prove that the nucleus containedneutral particles of the same mass as the proton calledneutron . 15. 16. Theatomic numberis the number ofprotonsin an atom.This number is unique for all elements and the atomic number is used to identify each element. Since atoms are electrically neutral,THE NUMBER OF PROTONS EQUALS THE NUMBER OF ELECTRONS. 17. Isotopes are atoms of the same element that differ in the number ofneutronsin the nucleus.Isotopes of the same element have the different chemical properties. The number of neutrons determines the particularisotopeof the element. 18. 19. Subatomic Particle Chart Particle Charge Relative mass Location Electron -1 1/1840 Outside Nucleus Proton +1 1 Nucleus Neutron 0 1 Nucleus 20. Themass number is the total number ofprotonsandneutronsin an atom. Mass Number=protons+neutrons Mass numberprotons=Number ofNeutrons 21. 22. 23. Calculating Atomic Mass Atomic Mass is theweightedaverage of the masses of theisotopesof that element.A weighted average takes into consideration both themassand theabundanceof each isotope.The correct unit for atomic mass is amu. 24. To calculate relative atomic mass, multiply themass number of each isotope by itspercentabundance changed to a decimal and total. ( Mass #)(isotopes relative abundance) + (Mass #)(another isotopes Rel. abund) = Relative atomic massof the element 25. EXAMPLE: 32.09384 amu Symbol Abundance Calculation Average Atomic Mass 32 S 95.00% 32 X 0.95 30.4 33 S 0.76% 33 X 0.0076 0.2508 34 S 4.22% 34 X 0.0422 1.4348 36 S 0.014% 36 X 0.00014 0.00504 26. Example 2:Neon has 2 isotopes, Ne-20 with an abundance of 90% and Ne-22 with an abundance of 10%.Calculate the average atomic mass of neon. 27. Example 3:Carbon occurs in nature as a mixture of atoms of which 98.89% have a mass of 12.00 u and 1.11% have a mass of 13.00335 u.Calculate the atomic mass of carbon. 28. Radioactivity Radioactivity was 1 st discovered by Antoine Becquerel, whena photographic platenever exposed toSunlight in his lab had become exposed.The only possible culprit was a nearby uranium salt sitting on the bench top. 29. History of Radioactivity The termradioactivitywas 1 stused byMarie Curiein 1898.Curie and her husband, Pierre, found that radioactive particles were emitted as either electricallynegativewhich were calledbeta particles ( )orpositiveparticles calledalpha particles ( ). 30. Nuclear Chemistry Nuclear reactionsare reactions that affect thenucleus of the atom. 31. Radioactivityis the phenomenon of radiation (particles and/or energy) being ejectedspontaneouslyby an unstable nucleus until it reaches a more stable arrangement. 32. Nuclear Stabilityis determined by theratioof protons to neutrons in the nucleus. 33. There are forces in the nucleus that oppose each other, the " Strong " force holding Protons and Neutrons to each other and theelectrostaticforce of protons repelling other protons.Under certain arrangements of protons and neutrons the electrostatic force can causeinstabilityin the nucleus causing it to decay. It will continue to decay until it reaches astablecombination. 34. Radioactive decayis the process by which the unstable nuclei lose mass and/or energy by emitting radiation.Eventually unstable nuclei achieve a more stable state when they are transformed into atoms of adifferent element . 35. This graph shows the stable nuclei inred . There are several things to notice: 36.

  • There are no stable nuclei with an atomic number higher than83or a neutron number higher than126 .

37.

  • The moreprotonsin the nuclei, the more neutrons are needed for stability. Notice how the stability band pulls away from theP=Nline.

38.

  • Stability is favored byevennumbers of protons and even numbers of neutrons. 168 of the stable nuclei are even-even while only 4 of the stable nuclei are odd-odd.

39.

  • Types of Radioactive Decay
    • When unstable nuclei decay, the reactions generally involve the emission of aparticleand orenergy .
  • For each type of decay, the equation isbalancedwith regard to atomic number and atomic mass.In other words, the total atomic number before and after the reaction are equal. And the total atomic mass before and after the reaction are alsoequal .

40. Transmutation When particles break down in the nucleus in an atom of an element (radioactive decay), the element changes into another element.This is calledtransmutation . 41. TYPES OF RADIATION Gamma emissionis the high energy electromagnetic radiation given off in most nuclear reactions.GAMMA RAYS ARE NOT MATTER, THEY ARE ENERGY. Therefore, they are not involved in balancing the nuclear equation.They are very damaging and difficult to shield against. 42. Gamma Emission ( ) Generally accompanies other radioactive radiation because it is the energy lost from settling within the nucleus after a change. 43. Alpha Emission ( ) Happens when the atomic number is greater than 83 The 2 p+ 2n() lossbrings the atom down and to the left toward the belt of stable nuclei. 44. Alpha Particle Emission ( ) Uranium - 238 Thorium - 238 Alpha Particle( ) 45. BETA EMISSIONA beta particle (a high energy electron, charge of -1) is generated in the nucleus as a neutron is converted into a proton. Carbon - 14 Nitrogen - 14 Beta Particle 46. Beta Particle Emission ( ) Happens to nuclei with high neutron:proton ratio A neutron becomes a proton causing a shift down and to the right on the stability graph 47. Positron Emission A positron is an antimatter particle that is like an electron but has a positive charge. A positron is generated as a proton is converted to a neutron. Fluorine - 18 Oxygen - 18 Positron 48. Positron Emission Happens to nuclei with a low neutron:proton ratio A proton becomes a neutron causing a shift up and to the left 49. This graph shows all the trends of decay and the band of stable nuclei 50. Penetrating Power of Radiation 51. Nuclear Fission Fission is the breaking apart of a veryheavy nucleus into parts. 52. A nuclear reactor is a device for controlling nuclear fission to produce energy for practical use.The main fuels for nuclear reactors areU-235and plutonium-239 . 53. Fusionis the combining of 2 small nuclei into 1 larger one.Fusion of hydrogen into helium occurs in the sun.Fusions reactions should produce much more energy than fission and use much more accessible fuels.However, currently many problems exist in fusion reactions such as the extremelyhightemperature needed for the reaction. 54. 55. Decay Series for Uranium - 238 56. Half-Life Half-Life (t 1/2 ) is the time required forhalfof the atoms of a radioisotope to emit radiation and to decay to products . 57. Half-Life Example It takes 4.5 X 10 9years for one half of a sample of uranium-238 to decay to lead-206.Therefore, it would take another 4.5 X 10 9years for one half of the remaining uranium to decay, et cetera, et cetera, et cetera. 100g 50g 50g 1 half-life 25g 25g 12.5g 12.5g 6.25g 2nd half-life 3rd half-life 4th half life 58. How many atoms of a 2.97g. sample of molybdenum-91 would remain after 62 min. if the half-life of molybdenum-91 is 15.49 min.? How many lives is this? Answer = 0.19 g4 half-lives # Half-Lives Time Spent (min) Amount Remaining (g) 0 0 2.97 1 15.491.49 2 30.98 0.74 3 46.470.37 4 61.96 0.19 59. Radiation Detection Film badgesare used to monitor the amount of radiation exposure people have received. 60. Geiger Counter Instrument thatdetects radiation by measuringcurrent produced by gas particlesionized byradioactivity 61. 62. Scintillation Counter Instrument thatconverts light to an electric signal for detecting radiation. 63. Uses for Nuclear Radiation Since the physical and chemical properties of radioisotopes of an element are the same as stable ones, many uses for radioactive nuclides are possible. 64. In medicine radioactive nuclides are used to destroy cancer cells and as tracers to tract substances through the body or identify cancer and other diseases. Cobalt - 60 Radioactive Tracer 65. 66. In agriculture, radioactive nuclides are used as tracers in fertilizer to determine the effectiveness or to prolong shelf life of food by irradiating to destroy microorganisms. 67. In dating radioactive nuclides are used to determine the age of objects.Example:Carbon -14 is used to date organic materials. 68. In energy production, currently nuclear fission is used to create energy.Example:Comanche Peak nuclear power plant in Glen Rose produces energy that is used by TXU. 69. Nuclear Waste Nuclear fission produces radioactive wastes that must be contained and stored on-site (temporary) or disposed of (permanent).