Introduction to Atomic Structure: Demonstrations and Labs Joseph D. Ciparick DeWitt Clinton H.S., Bronx. NY Atomic structure is often presented early in a chemistry course and then "confirmed" as the course nroceeds. I believe that more evidence is needed before adetailed structure of the atom is nro~osed,and I would offer the follouina sequence of labs and demonstrations to be included with the introduction of atomic structure. Establlshlng Electrical Properties: Somethlng Old, Somethlng New The purpose of this set of activities is to demonstrate a variety of electrical phenomena and tie them to the topic at hand. Caution must, of course, be used when handling electricity, and the teacher should be familiar with these activities before undertaking them in class. Equipment For all units except electrochemistry:electroscope, plastic, glass, and rubber rods; wool, silk, and other fabrics; pith balls, Van de Graaf generator, Tesla or induction coil, cathode ray tube, fluorescent tube, electromagnet,permanent magnet, dc motor, demonstration galvanometer, bimetallic strip, thermoelectric magnet, Geissler tubes. Geieer counter. Not all of this eauinment is essential, but . . most bf it &odd be available. Discussion and Demonstrations Students are asked what electricity does, and the demonstrations are carried out in resnonse to their comments: it produces light (both incandescent and fluorescent), operates electromagnets and motors, and is conducted by metals. When mentioning static charges and lightning, "static cling" and repulsion can he demonstrated with fabrics and the electroscope. A lightning discharge can be produced from the Van de Graaf generator, and sparks can be used to make the fluorescent tube glow. Students are usually interested in high voltage. I mention the fact that for electricitv . to eet throueh air about 20.000 volts per centimeter are needed. The Tesla or induction coil can he used to nrovide such marks. and. if a Levden - "iar is used, discharges can he strong enough to ignite paper or melt tin foil. The 1.eyden jar should be used only if the teacher is experienred in the methods uf charging and discharging it. To estahlish a link between these heno omen and a chemical reaction, a burning candle is placed near the Van de Graaf generator. The flame is deflected by the charge field. Finally, a flame from a match or smoke is used to discharge an electroscope. A smoke alarm works on this principle: a radioactive source charges the leaves, and when they are discharged by the smoke, an alarm sounds.
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Electrochemlstry: Labs and Demonstratlons Batteries will probably be mentioned as sources of electricity during these introductory demonstrations. Here is the opportunity for experiments in electrochemistry that are usually done much later in the course. These are qualitative demonstrations: different metals in an electrolyte to produce galvanometer readings and the observation of the results of electrolysis. For the latter, a solution of copper(I1) chloride is excellent.
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Journal of Chemical Education
Follow-Up Demonstrations Equipment Conductivity tester, electrolytic cell, 6-V dc source, carbon rods, magnesium ribbon, copper and zinc electrodes,standard flash bulbs for battery-operated cameras, nearly saturated solutions of eapnerfII) chloride.. conner(I1) nitrate. concentrated hvdroehloric acid. .. Erystn~~inr lithium chloridi, large evaporating diuh:~unsrnhurner; fi M nitric a d , connecting wire. electrode holderr. Procedure Using the conductivity tester, show the lack of conductivity of the dry salts and-the conductivity of a salt solution. Lithium chloride can he easily melted and the conductivity of a molten salt can he demonstrated. Conoer can then he slowly dissolved in nitric acid (if concentiied acid is used, the hood should be utilized). The solution can then be electrolyzed to produce copper at the cathode. A zinc strin connected with a wire to a carhon rod is then dipped into solution of Cu(NO& to which 25 mL of concentrated hydrochloric acid has been added. After a few minutes, thecarbon rod becomes coated with copper and the zinc reacts, releasing hydrogen and replacing copper. If a carbon rod is merely in contactwithzinc metal in any copper solution, it will be coated with copper. The magnesium ribbon is wound into a coil and attached with alligator clips to one connection on the flash bulb. A copper strip is connected to the other one, and both are dipped into a beaker of concentrated HCI. The bulb flashes.
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Discussion The students have observed several phenomena associated with electricity that they might not have been aware oT: a current is produced by a chemical reaction (galvanometer deflections and flash bulb discharges), and a current can cause a chemical reaction (electrolysis). Students are then asked if the phenomena are related in any way. The copper "disaoneared" in the nitric acid. the maenesium eventuallv disappeared in the flash bulb dkmonstrkion, and the zinc disappeared in the zinclcarhon cell. The flash bulb demonstration reinforces the galvanometer reading of the redox ex~eriments.An electric current is definitelv bv a . nroduced . chkmical reaction. The direction of the current is from the more reactive metal (galvanometer reading), so the electricity must come from the reacting zinc and magnesium. Copper must also he losing electrical charges when i t reacts in the nitric acid. Supplying electrical charges restores it to its original state on the carbon cathode. The same discussion can he followed for the nonmetals and the anions. Since the metals that disanoeared oroduce salts whose solutions conduct a current, 'tie students are asked what happens to the dry salt when it dissolves or melts. If the metal always appears at the negative cathode, can we say that there is an electrical attraction involved that is related to the attraction of static charges?
Eledrostailc Charges and Charge Flelds Equipment See list for fvst unit. Procedure Charge apith ball on a thread by touching it to the globe of the Van de Graaf generator. It will be repelled as soon as i t touches the globe, and will "bounce"off the edge of the field and never come near the globe. Students can almost "see" the field surrounding the globe in all directions. Threads taped to the globe will also suggest lines of force in the field. "Volta's hail" can also he demonstrated (pith halls in an inverted beaker over the globe). They bounce around vigorously. This equipment also comes with a random molecular motion demonstration kit. The cathode ray tube is then activated with a high-voltage coil. (X-rays are emitted from the tube so i t should be used for a short time only). The beam is deflected toward the Van de Graaf generator. The usual charge on the globe is said to be positive. An electromagnet is demonstrated together with the deflection of the beam by a magnetic field. In connection with the magnetic properties of a current, I also briefly demonstrate the dc motor and the reason why the galvanometer is deflected by a current. I t is also useful to demonstrate thermoelectric properties'by heating the bimetallic strip attached to a galvanometer and by demonstrating the electromagnet that is heated at one end and water-cooled a t the other. Not every physics department may have this equipment. Discussion Both the bouncing pith ball and the threads are used to illustrate the existence of a static field. The deflection of the cathode ray toward this field illustrates that it must be oppositely charged. If the field is positive, the ray must be negative. I then ask the students if they can see any relationship between the deflection of the ray and the magnetic field produced by a current. I lead the students through an analysis that concludes the cathode ray must he made of something that is negative and produces a magnetic field. I t comes from the metal and is pushed by the power source. Is it the same thing that came from the magnesium to flash the bulb? A compass held near the wire when repeating the demonstration is impressive. They have also seen the reason for the deflection of the galvanometer needle.
Toshow that thecathode ray beam is not just alight beam, I use a magnet with a laser beam. I have also used a magnet with a fluorescent tube and Geissler tubes, hut that can be deceptive. Even though there is no apparent deflection, positive charges are involved. A canal ray tube beam can be deflected. Does the electron come to the copper in the solution to turn it back into copper metal? Is the electron coming from the reacting electrode of the cell to where the copper appears? The Electrostatic Fleld and Ionic Crystals The field produced by the Van de Graaf generator is demonstrated once more and applied to the properties of an ionic crystal: each positive ion is "surrounded" by negative ions, each negative ion by positive ions. This makes the lattice bonding very strong. But the "exposed" ions a t the face of the lattice have a static field that is still able to attract the polar water molecules, which eventually remove them from the lattice, surrounds them in the solution, and allows them to wander without interacting. Since the anode and cathode reactions are not due simply to electrostatic attraction, I stress that the cathode and anode reactions depend on how strong a bond there is between the water molecules and the ions. Further discussion is put off until a more detailed treatment of redox potentials.
Summary What has been established is that reactions of metals ~ r o d u c ean electric current. Thev must be losing electrons. Electrons must heon thesurface hf t h e a t o m . ~ h &aresmall and mobile (cathode rav beaml.and can be ruhbedoff(static charge). Nonmetals gain e~ecirbns(anode reaction). periodic relationships can he introduced hv demonstratine how potassium is more reactive than sodium (a family relkionship) and how magnesium is less reactive than sodium, and calcium less reactive ~han.potassium (periodic changes). The mobility of outer electrons in metals is reinforced by the thermoelectric effect. Ions are demonstrated not only in electrolysis, but also in the neutralization of the electroscope by smoke particles, the deflection of the candle flame by the field around the Van de Graaf generator, and the visible sparks. All this is an introduction to the problem of atomic structure and reaction mechanisms. The students have been immersed in demonstrations and experiments that make some of the basic concepts more realistic. In the lone - run. . more questions than answers may result from these experiences. But that, after all, is what chemistry is all about.
Responsibility Experiments, laboratory exercises, lecture demonstrations. and other descriptions of the use of chemicals, apparatus, and instruments are presented in this Journal as illustrative of new, novel, or improved ideas or concepts in chemistry instruction and are directed at qualified teachers. Although every effort is made to assure and encourage safe practices and safe use of chemicals, the Journal of Chemical Education cannot assume responsibility for uses made of its published materials. We strongly urge all those planning to use materials from our pages to make choices and to develop procedures for laboratory and classroom safety in accordance with local needs and situations.
Volume 65 Number 10 October 1988
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