The Beginning of the Atom Democritus s Atomic Philosophy 460BC 370BC ~ Believed atoms were indivisible and indestructible Dalton took it and ran with it 2000 yrs later! Jan 31 3:33 PM Dalton and the Atomic Theory: circa 1800: Back in the day this is all they "knew" (or at least theorized). Atoms are indivisible (today known to be FALSE b/c know about subatomic particles!) In any sample of a pure element, all the atoms are identical in mass and other properties. (today known to be FALSE b/c know about isotopes!) Atoms of one element are different from atoms of another element (still true) Atoms combine in simple whole number ratios (called Law of Multiple Proportions) During reactions atoms are rearranged but unchanged (relates to Law of Cons of Matter) Jan 31 4:08 PM 1
Cathode Ray Tube (CRT) At low pressures, current flows through a gas. Experimentation shows: Glow on the glass opposite the cathode Different gases cause different color glows Paddle wheel in the middle spins bc bombarded with e- Conclusions: Cathode rays are composed of particles. Cathode rays are deflected by external electric and magnetic fields. Jan 31 4:09 PM Rays are deflected away from the negative pole of a magnet!) Therefore: particles are negative!!!!!!!!!!!!!!!!!! http://www.youtube.com/watch? v=7yhwmwcxex8&safety_mode=true&persist_safety_mode=1 Jan 31 4:09 PM 2
JJ Thomson (1897) ~ Concluded that cathode rays were beams of identical, negatively charged particles (he did all of the stuff explained before...) ~ Called them electrons ~ Studied deflection of cathode rays and found that: Heavier (subatomic) particles were deflected less with magnet > More charge = more deflection (why b/c higher repulsion!) > e- has mass and a charge; and that its mass-to-charge ratio was much smaller than that of the hydrogen ion > 2 particles with the same mass-to-charge ratio move in the same path in a vacuum when subjected to the same electric and magnetic fields. JJ's Model = Plum Pudding Model YUM!!!! Jan 31 4:12 PM Robert Milikan (1909)Preformed the oil drop experiment Drops fall due to gravity X ray put a negative charge on drops He measure charge on the drops Measured the amount of charge for each e then using Thomson s ratio calculated the mass of an electron This is what he did with a short explanation http://www.youtube.com/watch?v=xmfyhag7liw Jan 31 4:20 PM 3
Eugene Goldstein Modified a cathode ray tube Found positive charge in the atom (actually was first to discover the positive charge but never coined the term "proton") ~2000X heavier than the e- Jan 31 4:21 PM Ernest Rutherford (1912) performed the gold foil experiment (Figure 4.7 pg 107) shot positively charged alpha particles at a piece of gold foil Predicted that particles would pass through b/c atom is mostly empty space RESULTS: 99% did 1 % were deflected concluded that there was a concentrated positive center (w/all mass!) in the atom... later coined the term "proton" Jan 31 4:21 PM 4
Rutherford s model (and final conclusions): The positive center was called the nucleus Most of the atom is empty space e are outside of the nucleus Watch this! Summary of JJ Thompson & Rutherford Animations Jan 31 4:22 PM James Chadwick (1932) discovered the neutron suspected purpose is to hold the protons in the nucleus together (it is the peacekeeper of the protons!) Jan 31 4:22 PM 5
Niels Bohr: ~ Revised Rutherford s model to show electrons traveling around the nucleus in ENERGY LEVELS CALLED ORBITS!!!! ~ e s orbit the nucleus in SET PATHS Planetary model Feb 7 4:19 PM There are only 4 (visible) energy levels available to Hydrogen The e s are quantized (have a specific amount of energy) The spectra (light) lines are formed when an e is moving between 2 energy levels or orbits (see below, light energy released!) We will talk a LOT more about this so don't be too worried if it looks difficult... Spectra! Watch this! http://www.learnerstv.com/animation/animation.php?ani=124&cat=chemistry Feb 7 4:24 PM 6
Schrödinger : > e are quantized (like what Bohr said) > Does not define a specific path or orbit of an e instead e s are free to move around the nucleus (opposite of Bohr) > Estimates the probability of finding an electron in a particular region of space around the nucleus called an electron cloud. http://www.youtube.com/watch?v=uwmtorux0lm Feb 2 3:23 PM The Elements http://www.youtube.com/watch?v=zgm wskfbpo Jan 31 4:23 PM 7
The Atomic Timeline Work on your selected scientist! Follow the rubric online and think about 3D model Please be sure to include all of the info asked for! Historical Model of the Atom Project: You will be completing this in a group of 6-7 students, each person acting as one of these historical scientists! The following people were important scientists who worked on the question What is matter made of? Some of them made discoveries of subatomic particles, while others developed a model for the atom. Millikan, JJ Thomson, Chadwick, Rutherford, Dalton, Bohr, Schrodinger You are going to ACT as your chosen scientist while giving background information and facts about your scientific contributions to the evolution of the atomic model. Think of this as a memoir of yourself and your experience during your discovery of the atom! Part I: Be one with the scientist prepare for your presentation For your scientist, answer the following questions: Who you are (you may include an interesting fact about your career or life) What was your discovery or model (describe it FULLY) o What experiment was used? (may not apply to all scientists) o What were the results of the experiment? (may not apply to all) o What conclusions were drawn from the results? (may not apply to all) o What information you used from your predecessors? When the discovery was reported (year) and what was the reaction of the scientific community? Include a major worldwide event(s) occurring at the time the discoveries were reported Which parts of the model or discovery are known to be true today? ** For the areas in which the model was wrong, why were the wrong statements important? Part II: Visual representation of your atomic model. Create a 3D representation of your atomic model (student created 3D model or use Mrs. Enlow s 3D Doodler pens). ***3 of your GROUP S models must be created by 3D Doodlers (your choice which ones!) 3D model must be between 4-8 inches in diameter Identify the title of the model and the scientists that is was attributed by http://atomictimeline.net/index.php Label necessary areas of the model (e-, p+, nucleus, orbitals, etc) EXTRA CREDIT: Dressing up as your scientist for the presentation portion of the project. Jan 31 4:24 PM 8