Greetings Squaddies! Nice to see you again. I haven't mentioned it in some time, but I don't want you to think that I'm completely running the science show here. If you have a burning question, or some piece of the universe's grand puzzle is keeping you awake at night, simply leave it in the comments, or send me an email and we'll get working on it pronto. The email address is suburbanjoe - at - gmail - dot - com. I'm assuming you're smart enough to take that spam-unfriendly address and turn it into something useful on the intra-web. If not, well, I think we have our next post.
So, just who in the hell is John H. Van Vleck and why are we thanking him? Mr. Van Vleck was a Nobel Prize winning physicist who was primarily responsible for ligand field theory, an apect of which we're going to discuss today as we continue our journey through the various physical and chemical causes of color.
Before we can talk about ligand field theory, as it relates to color, that is, we have to talk about ligands. Our friends over at Wikipedia define a ligand as "an atom, ion or functional group that donates its electrons through a coordinate covalent bond to one or more central atoms or ions, usually metals." The important thing there is the donation of electrons. As you may remember from our previous discussion, when one is speaking of electrons, one is also speaking of energy levels, and we all know what happens when electrons start acting all crazy and messing around with their energy levels. That's right, color. Well, that and sweet, sweet, monkey lovin'.
Colored gemstones come about when an extremely hard mineral has a very small amount of impurity (usually around 1%) introduced into it. For example, lets look at corundum. Corundum, also known as Aluminum Oxide, is the second hardest substance known to you or I. When completely pure, corundum is colorless. Add some impurities, namely titanium and iron, and you get sapphires. Replace those impurities with chromium and you get a ruby. Chromium is also responsible for the green color of emeralds, however in the case of emeralds, the base mineral is Beryl, or Beryllium Aluminum Silicate. The difference in color, as a result of the difference in the reaction between chromium and corundum and between chromium and beryl, can be explained with ligand field theory. You had to know we'd get there eventually.
Corundum, by itself, is a somewhat messed up octahedron, with a chewy center of aluminum ions surrounded by oxygen ions. In this structure, all of the electrons are paired off, which means they're not free to jump energy levels, so no light is absorbed. Hence the colorless quality. Now, lets say that we replace 1% of those aluminum centers with chromium instead. Chromium by itself has 3 unpaired electrons, but they all occupy the same energy level, so again, no energy is absorbed and no color is produced. However, when chromium is introduced into the structure of corundum, the surrounding oxygen ions, our ligand field in this case, create such a tizzy that the normally staid energy level of the three unpaired chromium electrons is split into 2 different energy levels. The three unpaired chromium electrons are now free to jump between the 2 energy levels. And how do they do this? That's right, by absorbing light. The new split in energy levels is such that the chromium electrons can absorb both green and violet light. With these parts absorbed, that leaves the red part of the spectrum to shine through, hence a ruby's red color. When chromium interacts with beryl, the resulting energy level split is smaller, so the electrons can absorb the less energetic part of the spectrum, namely yellow-red and blue. As a result, the green light is allowed to come through, hence the emerald being the gem of choice for Leprechauns everywhere. Actually, that's not true. Everyone knows that the gem of choice for Leprechauns is the rare Purple Horseshoe.
It is important to note that while we're still dealing with electrons jumping energy levels, as with last week's post, the means of color generation as a result is different. In gas excitation, energy, usually electrical energy, causes the electrons to jump a state. They're not particularly happy about this, so when they come back down, they emit radiation in the visible spectrum. When speaking about gemstones, it is the light itself that is absorbed by the electron, which in turns allows for the jump in energy levels, a situation these electrons seem quite happy with. When the electrons absorb some of the light, the remaining parts of the spectrum is what then makes it to your eye. See, in one case, the electrons are emitting radiation in the visible spectrum (Neon signs) and in the other, the electrons are absorbing radiation in the visible spectrum (Emeralds). Emeralds and rubies aren't the only gems that get their color from the ligand field effect. Garnet, topaz, torquoise, tourmaline and many other gems that you've never heard of, but your significant other no doubt wants bestowed upon them as gifts, all get their color from the interaction between impurities and their base minerals.
It's all so fascinating I can barely stand it. Remember, when someone calls you dirty and impure, perhaps for your fascination with naked hamsters, remember that it is these impurities that allow your true colors to come shining through, like the dazzling ruby and the beautiful emerald. Except for you. You're just sick.
Next week: Molecular Orbits and You, Perfect Together
Extreme Science - Gemstones
WebExhibits - Causes of Color
Wikipedia - Ligand Field Theory
Wikipedia - Ligand
BookRags.com - Ligand Field Theory
Encyclopædia Britannica Online - Ligand Field Theory
Encyclopædia Britannica Online - John H. Van Vleck