Atomic Experiments Regular Fall07
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Atomic Experiments Regular Fall07

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History of atomic theory through Rutherford's model of the atom.

History of atomic theory through Rutherford's model of the atom.

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  • Thanks for this slide show, it was interesting to understand, I knew some, but it was very refreshing and more indepth than just saying 'there is a nucleus with electrons spinning around it'. Plus for further mysteries to be solved, we need to understand the basics, its one link after another till the bridge is crossed.
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Atomic Experiments Regular Fall07 Atomic Experiments Regular Fall07 Presentation Transcript

  • Why is learning about the atom important….?
    • The atom was considered to be an “imaginary” thing, something that people laughed at!
    • Roosevelt trusted Einstein, and funded his secret research known as the “Manhattan Project”
    • In a few short years, the idea of an invisible atom had been developed into a city-destroying weapon - the atomic bomb!
  • In Hiroshima, of a resident civilian population of 250 000 it was estimated that 45 000 died on the first day and a further 19 000 during the subsequent four months. In Nagasaki, out of a population of 174 000, 22 000 died on the first day and another 17 000 within four months. Over 90% of each city was destroyed.
  • Early Atomic Ideas
    • Democritus (5th c. BC): matter was made up of things that were indestructible, unchanging & different
    • atomos = indivisible
    • Aristotle: Didn’t believe in atoms!
  • Dalton’s Atomic Theory
    • TOOK WORK OF PREVIOUS SCIENTISTS & ASSEMBLED IT: (5 postulates)
    • All matter is made up of atoms
    • Atoms are indivisible (Democritus)
    • Atoms cannot be created or destroyed, just rearranged (Lavoisier)
    • Atoms combine in whole number ratios to form compounds (Proust)
    • Atoms of one element are identical to each other, but different than atoms of differing atoms
  • The Early 1800’s!
    • Lots of introductory work with electricity
    • Matter is electrical in nature!
    • Ben Franklin assigns + & - charges to charged objects (didn’t know it had to do with the atom!)
  • But technology was limited until the late 1800’s, early 1900’s….
    • With technology comes experimentation!
    • With these experiments, what part of the atom do you think was the first to be discovered…..?
    ………… ..THE ELECTRON! Why do you think it was the first to be discovered……?
  • Cathode Ray Tube, or Crooke’s Tube
    • Evacuated tube containing two metal electrodes or plates connected to a battery
    • William Crookes designed this tube in 1879
    • The cathode ray inside glows green
    • Crookes was convinced this beam consisted of charged particles
  • So what are Cathode Rays….?
    • J.J. Thomson, in 1897, answered this question!
    • Two charged plates were used above and below the beam
    • The Beam was attracted to + plate, and was deflected from – plate!
    • CRT Demo
    • Conclusion: cathode rays were composed of negatively charged particles called electrons!
  • The cathode ray tube was the precursor to the modern day….. TELEVISION! Black and White….. Color…….
  • Millikan Oil-Drop Experiment
    • Robert Millikan in 1909 discovered the charge and mass of an electron
    • He placed electrically charged oil drops in an electric field
    • The oil drops were sprayed into the upper chamber
    • Some drops gained – charge (static electricity) & were repelled by the - plate
    • Millikan Oil Drop Demo
  • Now that we have found negative particles….
    • Eugen Goldstein in late 1800’s used a Crooke’s tube with holes in the cathode and discovered positive particles
    • He shot a cathode beam (beam of electrons) at hydrogen atoms
    • The electrons from the atoms went with the beam of electrons to the + plate
    • He also observed + particles moving backwards towards the + plate and going through the holes!
    • Different amounts of + particles were produced when different gases were used because each element has a different number of protons!
    • Canal Rays Demo
  • Who puts all of this information together into “The Big Picture”?
    • J.J. Thomson theorized that these negative and positive particles were actually part of matter - part of the atom!
    • He develops the first working model of the atom, known as the plum pudding model!
    • Dalton’s idea that atoms were solid particles like marbles that weren’t made of smaller pieces was…REJECTED!
  • Why was any of this monumental…?
    • People didn’t believe in atoms, and didn’t understand what things like electricity and light were!
    • Thomson showed that electricity was nothing more than a flow of little particles called electrons!
    • And electrons, along with protons, make up every atom, in every person, plant, building and object in the universe
    • We are made of charged particles - the same particles that we use for electricity
  • So atoms are made of + and - particles. How are some atoms “radioactive”? What is “radioactivity”?
    • Henri Becquerel , in 1895, put a chunk of Uranium in a desk drawer with some photographic paper
    • Becquerel knew the Uranium atoms must be giving “something” off. He called this something “radioactivity” or “ radiation ”.
    • The same year, William Roentgen had a similar experience - his Uranium actually caused photographic paper to develop in a nearby lab - through a wall!
    • He called his rays, “ X-Rays ”. They were immediately used for medical purposes!
    • What was this “radiation”, though? How did it relate to what Thomson had discovered?
  • Ernest Rutherford was determined to figure out what this “radiation” was!
    • Ernest Rutherford, a physicist from New Zealand, took a chunk of uranium, and set out to determine what it “gave off”!
    • He placed the Uranium in a lead box
    • He took two charged plates, and placed them above and below the beam of “radiation” that the Uranium was giving off
    • It split into three beams!!!
    • Radiation scattering demo
  • Rutherford’s Radiation Experiment
  • In Summary, No mass, but speed of light! Can be dangerous! 0 0 Gamma  Fast, but no mass so little energy -1 0 Beta (  Slow and no energy +2 4 Alpha (  Speed and Energy charge Mass (amu) Type of radiation
    • Atoms, we know, contain protons, neutrons, and electrons
    • Up until this point, only protons and electrons were known!
    • Why did some atoms give off radiation?
    • To answer this question, we have to look back at the structure of the atom!
    Why do atoms give off radiation, then?
    • People still accepted Thomson’s model of the atom!
    • His model was wrong though!
  • The Gold Foil Experiment…. What is happening? Why are the alpha particles bouncing off?
  • Gold Foil Experiment Rutherford's Experiment
  • What is going on….?
    • The alpha particles are bouncing off of something!
    • What charge does an alpha particle have?
    • What could it be bouncing off of?
    • They must be bouncing off of something that is….
    • Positive!
    • And big!
    • Rutherford theorized that all of the protons weren’t scattered about, but were all concentrated into a dense center, which he called the nucleus!
    • The electrons must be orbiting around this nucleus! Why?????
  • Why did he think that ALL the protons were in the center?
    • Alpha particles are huge!
    • They contain 2 protons and 2 neutrons!
    • They have a positive charge, because neutrons have no charge!
    • Like charges….
    • REPEL
    • But one proton wouldn’t have enough force to push away an alpha particle coming in!
    • He knew that there must be MANY protons in the center blocking the alpha particles!
  • Rutherford’s “nuclear” model!
  • What is the relative size of the atom….?
    • An atomic model the size of Busch Stadium and parking would contain a pea sized nucleus containing 95.95% of the atoms mass.
    • The pea at the pitcher’s mound would be the nucleus, and an ant crawling on the parking lot outside would be an electron!
    • The atom is mostly…..
    • Empty space! Nothing!
  • So why do some atoms give off radiation????
    • Rutherford looked at his model of the atom for answers!
    • All of the protons are jammed into a tiny little nucleus!
    • And they all have what charge….?
    • The same charge! Positive!
    • Which means…..
    • They are constantly trying to push each other out! Like charges REPEL!
    • What keeps them in there…..?
  • ……..The neutron does!
    • The neutron was the last particle to be discovered!
    • Why was it the last particle to be discovered….?
    • IT HAS NO CHARGE!
    • It was discovered when James Chadwick noticed that Beryllium atoms were giving off some “unknown particle” when hit with alpha radiation!
    • These particles were neutrons!
    • Why are the neutrons even in the nucleus?
    • They have a special force, known as “STRONG NUCLEAR FORCE”, that holds the protons together, because the protons are always pushing each other out!
    • I call it “NEUTRON GLUE”!
  • Problems after element 82…..
    • After lead, element 82, there are too many protons in the nucleus, and no number of neutrons can hold in the protons
    • The atom has gotten too big, and begins ejecting stuff to release the pressure!
    • This stuff is called radiation, or radioactivity!
    • Radioactivity is when an atom has gotten too big and it gives off stuff to relieve its pressure
    • Most atoms normally have more neutrons than protons anyway to keep the protons in place - atoms need added reinforcement!
    • So, any atom after lead is constantly giving off radiation to relieve its pressure!
    • What kind of “stuff” could an atom give off if it has gotten too big?
  • In summary…..
    • We know that the nucleus contains…
    • Protons and neutrons!
    • And the electrons are…..
    • Orbiting around the outside!
    • Atoms that have too many protons in the nucleus (more than 82) will give off…
    • Radiation! To relieve the pressure!