structure of the atom. what you already know about the atom nucleus contains protons (+) and...

Download Structure of the Atom. What you already know about the atom Nucleus contains protons (+) and neutrons (neutral) Electrons (-) orbit the nucleus in “shells”

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  • Slide 1
  • Structure of the Atom
  • Slide 2
  • What you already know about the atom Nucleus contains protons (+) and neutrons (neutral) Electrons (-) orbit the nucleus in shells 1 st shell: 2 e - 2 nd shell: 8e -
  • Slide 3
  • Daltons Atomic Theory (1808) Which of Daltons four postulates do we believe to be correct today? 1.All matter is made up of small indivisible particles called atoms. Atoms cannot be created or destroyed. (F: The atom was split in W W II.)
  • Slide 4
  • 2.Atoms of one element cannot be converted to atoms of another element. Too bad for alchemists. (F: Particle accelerators can convert the nucleus of one atom into that of another atom. This is nuclear chemistry and does not involve chemical change.)
  • Slide 5
  • 3.Atoms of a particular element are identical in terms of mass, size, etc. Atoms of each element have unique properties. (F: Existence of isotopes. An isotope of an element has the same number of protons in the nucleus, but different number of neutrons.
  • Slide 6
  • Isotopes of Carbon Isotope# p + #n o mass # 12 6 C 6612 13 6 C 6713 14 6 C 6814
  • Slide 7
  • NOTE: The different macroscopic forms of carbonand some other elementsare called allotropes. The allotropes of C are: diamond graphite buckyballs nanotubes.
  • Slide 8
  • 4.Atoms combine in specific proportions small whole number ratiosto form compounds. (T: Consider NaCl, H 2 O, H 2 O 2, CH 4, C 6 H 12 O 6,)
  • Slide 9
  • History of Modern Atomic Theory 1.Discovery of the electron 2.JJ Thomson determined mass to charge ratio for the electron 3.Millikans Oil Drop Expt (mass of e - ) 4.Discovery of the proton 5.Thomsons Plum Pudding Model of the atom 6.Rutherfords Gold Foil Experiment (discovery of nucleus)
  • Slide 10
  • 7.Mass SpectrometerAtomic Mass 8.Discovery of the Neutron 9.Electron Configuration (next slide show)
  • Slide 11
  • Discovery of the Electron Sir Wm Crookes (1832 1919) A man of science. Discovered thallium in 1861. Worked on pure and applied science, economic and practical problems, and psychic research.
  • Slide 12
  • Cathode Ray Tube Crookes did a lot of work with a CRT
  • Slide 13
  • Schematic Diagram of CRT
  • Slide 14
  • The inside of the CRT had a phosphor coating, that gave off light when struck by the beam.
  • Slide 15
  • Crookes placed an electric field just outside the CRT. He noticed that the beam was bent towards the external (+) charge.
  • Slide 16
  • Crookes placed a Maltese cross in the path of the beam. He observed that the shadow of the beam fell on the anode.
  • Slide 17
  • What Crookes Observed
  • Slide 18
  • What did these two observations suggest? 1. That the CRT beam had a (-) charge; 2.That the beam emanated from the cathodethe (-) electrodeand traveled to the anode (+). Theres more...
  • Slide 19
  • Crookes placed a paddlewheel in the CRT. The beam caused the paddle wheel to turn.
  • Slide 20
  • Crookes coated the tips of the paddle with a phosphor. They glowed when struck by the CRT beam. PrnY
  • Slide 21
  • This indicated that the beam had mass. ie. The beam was made up of (-) charged particles.
  • Slide 22
  • Same behaviour observed when the CRT filled with different gases (all at low pressure) and when different metals used for the anode and cathode. This suggested that the (-) particle that made up the beam was common to all elements. This particle is called an electron.
  • Slide 23
  • Cathode rays path was also bent by external magnetic field.
  • Slide 24
  • British Physicist JJ Thomson also experimented with a CRT. He varied external magnetic and electric fields to determine the charge to mass ratio of an e - to be 1.76 x 10 8 C/g. (The coulomb (C) is the SI unit for electric charge.)
  • Slide 25
  • Another Diagram of Thomsons Apparatus
  • Slide 26
  • Millikans Oil Drop Experiment In 1909 Robert Millikan (1868-1953) of the University of Chicago measured the charge of an electron.
  • Slide 27
  • Apparatus for Oil Drop Experiment
  • Slide 28
  • Explanation of Oil Drop Expt Small drops of oil, when zapped with x-rays, picked up extra electrons. Charged oil drops were allowed to fall between two electrically charged plates. Millikan monitored the drops, measuring how the voltage on the plates affected their rate of fall. From these data, he calculated the charges on the drops. His experiment showed that the charges were always whole # multiples of 1.60 x 10 -19 C, which he deduced was the charge of a single electron.
  • Slide 29
  • Millikan then calculated the mass of the electron by using his value for the charge, 1.60 x 10 -19 C, and Thomson's charge-to- mass ratio, 1.76 x 10 8 C/g: 1.76 x 10 8 C = 1.60 x 10 -19 C e - charge 1 g? g e - mass ? = 9.10 x 10 -28 g is the mass of one e -. This is ca. 2000 X lighter than a H atom.
  • Slide 30
  • Discovery of the Proton Eugene Goldstein made a CRT with a perforated cathode.
  • Slide 31
  • Canal Rays He noticed a beam traveling in the opposite direction from the cathode rays (violet glow). He called these canal rays.
  • Slide 32
  • While the beam of electrons could be deflected by an external magnetic field, the canal rays are barely affected. The canal rays are composed of protons (+). If low pressure H 2 (g) is in the CRT, heres what we have: H 2 (g) + E(from e - beam) 2H(g) 2H + E(from e - beam) 2H + + 2e - protons travel towards cathode (-) (protons are much heavier than e - s)
  • Slide 33
  • Another diagram of CRT with perforated cathode
  • Slide 34
  • Fun with a Crookes tube This demonstrates the phenomenon of discharge at different pressures of gas inside the tubes.
  • Slide 35
  • Thomsons Plum Pudding Model of the Atom The discovery of the electron lead to a simple model of the atom. Electrons randomly distributed in a positively charged pudding. Seemed to make sense at the time.
  • Slide 36
  • Mass Spectrometer In 1920s, F.W. Aston developed the mass spectrometer. This allowed the determination of atomic mass. And it showed a problem. Helium, for example, was observed a mass of 4 amu, not 2, as suggested by its 2 protons.
  • Slide 37
  • What was missing from the atom? The neutron. Discovered in 1932. Why did it take so long? Neutrons are neutral and dont respond to electric or magnetic fields.
  • Slide 38
  • When James Chadwick shot alpha particles at beryllium (atomic number 4) the beryllium emitted a neutral radiation that was later determined to be a stream of neutrons. So now we have the three subatomic particles: protons neutrons electrons.
  • Slide 39
  • Rutherfords Gold Foil Expt Rutherford and his co-experimenter Ernst Geiger shot a beam of alpha () particles through a thin sheet of gold foilonly a few thousand atoms thick. particle = He 2+ (He nucleus) mass of particle = 4 amu
  • Slide 40
  • Rutherfords Apparatus
  • Slide 41
  • Based on the Plum Pudding model of the atom, what would you expect if an particle (4 amu) was shot at a proton (1 amu)? Rutherford expected the particle to pass straight through the foil.
  • Slide 42
  • What Rutherford Observed prediction based on P.P. Model what was actually observedsome particles defected or even bounced back!
  • Slide 43
  • Interpretation of G.F. Expt Atom has a very small, dense core contains protons and neutronscalled the nucleus. We now know that the nucleus fills about a billionth of the atoms volume.
  • Slide 44
  • Slide 45
  • Of his experimental observations, Rutherford said: It was as if you fired a 15 inch shell against a piece of tissue paper, and it bounced back at you.
  • Slide 46
  • Weve accounted for the subatomic particles and the nucleus. But where are the electrons? In shells, you say? Its a little more involved than that... Stay tuned.


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