CONTENTS... Fluorescence...Coloring Fire...Emission Lines...Helium

Rationale: There is a difference between exploration and experimentation. Both are fundamental to the advancement of SCIENCE. However exploration must be done before experimentation with controls, 'experimentals', variables, etc., can be done. As this is being written, earthlings are beginning to explore Mars and other planets. We don't say that we are experimenting with Mars. We are only at the beginning - 'exploring.'

INTRODUCTION: One of the most difficult things for beginners to comprehend in chemistry are the subatomic particles. We explore things that are so small and weigh so little that they are almost not there, yet their effects when in partnership with zillions of others fill our lives. These effects are the basis of CHEMISTRY, which is the interactions of electron orbitals. Your teacher probably has some styrofoam, or gum-drop atoms or molecules around, and perhaps even has some atoms with some 'electrons,' and has been telling you that the electrons revolve around their nuclei just as the planets go around the sun. These thoughts are very good conceptual models invented by a Danish physicist Niels Bohr shortly after the opening of the 20th Century. Hence, we call these the Bohr models of atomic structure. We are now going to check out some of those ideas.

Exploration ONE: Fluorescence.

PUT ON EYE PROTECTION!* Turn out the room lights and shine the UV light around on various things. What's really nice to have is a collection of various minerals and rocks. Many glow (fluoresce) in brilliant fuchias, greens and blues. If you have a "short wave" UV lamp, you will not be able to see any glow from the lamp itself; you'll only see the fluorescing rocks and minerals. This is because the wavelength of the UV light (more violet than the deepest violet you can see) is too short to stimulate anything in your eyes (many insects, however see in that part of the spectrum: many flowers that are white to us, and ultraviolet to the insects). Your physics teacher will show you equations that tell you that UV light is a much higher energy than visible light.

Thus when the UV hits the rocks, those high-energy UV-photons stimulate the electrons in some of the rocks and bump them out of their "planetary" orbits. The electrons then like to fall back into their original orbits. And when that happens, a photon of visible light is emitted, and, since energy runs downhill, the emitted photon must have lower energy than the stimulating photon. We thus sent in invisible high-energy photons, and got back lower-energy visible photons. As astronomers might say, we have moved "redder", or that each step down gives us fluorescence that is closer to the red end of the spectrum - lower energy.

Exploration TWO: Flame-photometry.

In doing the following directions, you will be using some large crystals or chunks of these three salts: LiCl, NaCl, and KCl. They are going to be heated in a flame and some atoms will evaporate and their "hot" electrons will fall back into their proper places, and in so doing they will emit a characteristic color in the flame. Instead of using a higher energy UV light to excite the electrons in the salt, you are using heat energy. You are seeing fluorescence.

You know that Na⁰, for example, has a single electron in its outer shell. When that electron is ejected because of the heat, a moment later in cooler parts of the flame, the electron falls back and emits its particular color (wave-length) of light. (The Cl⁰ meanwhile might also be fluorescing, but in a invisible part of the spectrum, so we cannot easily explore that aspect, which is good - as we are allowed to easily explore the Na⁰!)

PREDICTION. By looking at your local Periodic Table of the Elements, how do Li, Na and K differ? (Just in case you were ever wondering, in many parts of the world these elements are called lithium, natrium and kallium - that's why the "Na" and "K".) You will note that each of these has a single electron in its outer shell, but that each has a different number of inner shells - Li has one inner shell, Na has two, and K has three inner ones.

LAB PROCEDURES

The following directions have three parts - the location, the spectroscope and the exploration.

1. LOCATION: find a dark place where you can set up the Meeker burner, but do NOT turn the burner on yet.

2. SPECTROSCOPE
   1. Focus the spectroscope
      1. Close the side-port by turning the objective. The internal splitter prism should no longer be seen.

      2. Look through and aim the spectroscope at a fluorescent ceiling light fixture. If you cannot see any light, turn the silver slit-width disk.
         
      3. Point the spectroscope at or near to the sun (it will not burn your eyes). This works on cloudy days, too!

      4. You are in good focus IF you can now see many very faint black lines cutting across the rainbow. If you don't, narrow the slit (picture gets dimmer), and fine-tune the focus by adjusting the eyepiece.

3. OBSERVATIONS
   1. SCHEME. Go from higher elements to lower ones as this helps eliminate cross-contamination when switching samples. The emissions get brighter as the elements get lighter (lighter and bright - I wonder why?).

   2. OBSERVATIONS
      0. Run water through the barrel of the burner to rinse off any residual salts of preceeding tests. Pay especial attention to rinsing the grill at the top of the barrel. Fling excess water from barrel; reattach to base; light burner for a few seconds to dry it; then turn it off.

      1. Place a chunk of the fused salt on the center of the burner's grill.
         
      2. What color do you see with the unaided eye? (violet? blue? faint blue? green? etc.)

      3. Look at the upper part of the colored flame using the spectroscope. What emission lines do you see?

      4. Rinse out the barrel of the burner and observe the next salt's emissions bands.

Back to astronomy and space travel!

Suppose you have a small asteroid that you want to move around with the rocket that you have strapped to it. If you want to move closer to the sun, you aim the rocket so that it will slow down your little planet, and if you want to move further from the sun, you speed up your asteroid. Again, inner orbits are lower energy, and outer orbits are higher energy.

So-, to eject Li's outer electron, which is only in shell number 2 should be much easier to do than to eject Na's electron, which is easier than ejecting K's electron. So when those electrons fall back into their assigned places, what colors do you expect to see? Would K's be bluer or redder than Na's?

Test it out by putting the chunks onto your burners!

Exploration THREE: SPECTROSCOPY

Just how discrete and unique are those colors? Quantum theory states that the electrons sit in very specific places and there are no halfway places. Look through your spectroscope (after you have focused it, of course**), and see if the colors form very narrow bands within the background rainbow, or are they broad bands? Those bright bands are called "emission" bands.

Look very carefully at the emission spectrum from Na. The orange flame consists really of how many colors. Look very closely. Do you really see one orange band? You see two?! How is it that you can see two? Does this mean that there are two types of Na atoms?

Do you see any other faint emission bands in sodium? What might they mean?

Now look at Li and K. Does Li's emission spectrum look anything like that of Na only "redder"; and does K's emission spectrum look anything like that of Na only "bluer"?

Exploration FIVE: STARS

What are stars made of? The answer to this ought to be leaping out at you - 'emission lines!' Unfortunately, if you look at the sun with a spectroscope (no, you don't burn your eyes out!) you see a rainbow. That is because the surface of a star is so hot that most atoms have more than just the electrons in their outer shell ripped off. The sun doesn't fluoresce, instead it incandesces. This is very much like the rainbow you get by looking at a light bulb.

BUT, look closer at the sun. You should not see emission bands but instead you should see absorption bands - very thin black lines that go perpendicular to the background rainbow. These or the opposite of emission bands. When white light goes through a gas, the converse of an emission spectrum is seen. Decades ago, scientists were able to account for all those black absorption lines - all except for two of them. Because the "color" is indicative of energy, they could accurately predict just what that element must look like. It was 2 protons and one or two neutrons - the second element on the periodic table. At that time the square was blank because no one had ever seen that element before. But there is was on the sun! So why not name it after Helios, the Sun? Hence, we have helium. Since that time He has been found in commercial quantities in natural gas as the radioactive decay process of K-40. Amarillo Texas is the Western World's main supply source.

* Eye protection. Make sure that the safety glasses that you are using are really able to block UV light. Test by placing a lens between the UV lamp and a fluorescent rock. The fluorescence should cease telling you that no UV is going through the lens.

**Focusing the spectroscope. As easy way is to aim the spectroscope at a fluorescent tube in a ceiling fixture. Go in and out with the eyepiece to find where the mercury emission lines are the sharpest. Then use that focus for all subsequent explorations.

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